Given Belers' political adherence of the 1320s, it has often been assumed that his murder was intended as an indirect attack on the powerful and wildly unpopular Despensers. Although this is certainly not impossible, the Sempringham annalist and the rather later Lancastrian chronicler Henry Knighton both say that Belers was on his way to dine with Thomas of Lancaster's brother Henry, earl of Leicester at the time of his death. Henry of Lancaster was certainly no friend of the Despensers, to put it mildly, even though his late wife Maud Chaworth, who died in about 1321, was the younger Despenser's older half-sister. Henry's appointment as one of the men ordered to bring the perpetrators to justice (see below) would tend to confirm that the chroniclers are correct, and that he had remained on good terms with Belers, despite the latter's change of allegiance.
Edward II, 120 miles away in Norwich at the time, heard the news of Roger Belers' murder five days later on 24 January, and appointed three men, his household steward Thomas le Blount, Henry Ferrers and John Hamelyn, "to make inquisition in the county of Leicester touching all persons concerned in the killing of Roger Beler, when he was going from Kirkeby to Leicester, and to arrest all those found guilty herein."  These three men were former (and future) Lancastrian adherents. The next record I can find is on 19 February, when Richard Perers, sheriff of Essex and Hertfordshire, was ordered to arrest "William son of Thomas la Zouche, knight, the brothers Folevylle and others guilty of the death of Richard Beleer."  On 28 February, Henry of Lancaster, earl of Leicester, Thomas le Blount (Edward II's formerly Lancastrian household steward), John Stonore and John Denum were given a commission of oyer et terminer ('to hear and determine') "touching the persons indicted of the death of Roger Beler, killed in going from Kirkeby to Leycestre, by an inquisition lately made by John Hamelyn and Henry de Ferers...". 
The next day, 1 March, Edward II's good friend and supporter Donald of Mar and another ten men including Edward's former household steward Richard Damory (elder brother of his late favourite Roger Damory) were appointed to "follow and arrest" the killers and bring them before Henry of Lancaster and his associates.  The men were now more fully named as: Ralph son of Roger la Zouche of Lubbesthorpe, Leicestershire; Roger la Zouche, son of Roger la Zouche, lord of Lubbesthorpe, knight; the brothers Eustace, Robert, Walter and Richard Folville, the latter parson of the church of Teigh in Leicestershire; Robert de Helewell, knight; Ivo son of William la Zouche of Haringworth, knight; Adam de Barley; William de Barkeston of Bitham; Robert son of Simon Hauberk of Scalford. On 14 March the justices of Wales and Ireland, the earl of Arundel and John Darcy, were appointed to pursue and arrest the men, some of whom were believed to have fled into Wales, and the sheriff of Leicestershire Edmund Ashby was ordered to arrest Thomas Folville, another brother, charged with aiding his brother Eustace, Ralph la Zouche and unnamed others of their gang to flee abroad and thus escape justice.  Finally, on 18 March John Denum and two other men were appointed to arrest the eleven men already named, and two others were added: John and William Stafford. 
Roger Belers' murderers were mostly or all local, from Leicestershire, the county where he was murdered and also where he held the majority of his lands. The seat of the Folvilles, Ashby Folville, is only five miles from Belers' (see below). Given this, and given the way Belers was still close enough to Henry of Lancaster to be invited to dine with him at Leicester and that many of the men charged with finding his killers were Lancastrians, I find some kind of local feud or disagreement between Belers and the thirteen men named as his killers a more convincing explanation for his death than vague notions that the murder was in some way intended as an attack on the two Hugh Despensers. Belers' seat was at Kirby Bellars near Melton Mowbray, where he founded an Augustinian priory in 1316; Rearsby, where he was murdered, lies five miles along the road from Kirby to Leicester, so there seems to be no reason to doubt that he was indeed riding there to meet Henry of Lancaster, earl of Leicester when he met his killers, as stated in several entries in the chancery rolls and in two chronicles. Belers left a widow named Alice and sons named Roger and Thomas (the latter perhaps named after the late earl of Lancaster, though of course I'm only speculating there); his elder son was still under age, i.e. under twenty-one, in June 1327. 
|The area of Leicestershire in question, from Google Maps. Leicester is to the south-west.|
Edward II and plenty of other men made strenuous efforts to capture those responsible for Roger Belers' murder, though to no avail: they all, as far as I can tell, fled either to Wales or to Roger Mortimer in France, where they returned with his invasion force in the autumn of 1326. It seems that Edward knew that some of them had joined Mortimer, as several days after the invasion, on 28 September 1326, Belers' killers were specifically excluded from a proclamation pardoning felons who would join the king against Mortimer.  Mortimer repaid some of them with pardons for the murder as soon as he was in a position to do so: on 11 February 1327, the Folville brothers Robert, Eustace, Richard and Walter, Adam de Barley and William de Barkeston were pardoned for the murder, along with three men whose names I haven't otherwise seen connected with the death, John Lovet, Thomas Alberd and William de Larketon. Maybe Mortimer had information that Edward II and his commissioners hadn't found. Sir Roger la Zouche of Lubbesthorpe was pardoned on 20 February 1327 for Belers' death and also for "breaking prison at Leicester," which implies that he had been temporarily captured after the murder and perhaps that one or several of his associates, maybe Thomas Folville who was accused of aiding some of the gang to flee abroad, had been instrumental in this. At around the same time Mortimer appointed three men, including John Denum, "to hear and determine the inquisition and indictments, returned by Thomas le Blount, John Hamelyn and Henry de Ferers, touching the death of Roger Beler while going from Kirkeby to Leicester."  What he was hoping to find or to achieve, having pardoned most of the men guilty of the murder, I don't know. The three men named in 1327 increases the number of those involved in Belers' death to sixteen; although he apparently had a retinue of fifty men with him, they were unable to save him. The chronicler Henry Knighton lays most of the blame for Belers' stabbing on Eustace Folville, the second-eldest of the seven Folville brothers (the eldest, John, did not participate, and neither did Laurence, as far as I can tell).
A monument known as the Folville Cross is said to mark the spot of Roger Belers' murder. For the Folville brothers, it was their first major crime, but certainly not their last: until the early 1340s they terrorised the English Midlands as one of the most notorious criminal gangs of the era.
1) Calendar of Patent Rolls 1317-1321, pp. 228, 335; the entry on Belers by Jens Röhrkasten in Oxford Dictionary of National Biography.
2) Patent Rolls 1321-1324, p. 189; Röhrkasten, ODNB.
3) Patent Rolls 1324-1327, p. 238.
4) Calendar of Chancery Warrants 1244-1326, p. 575.
5) Patent Rolls 1324-1327, pp. 283-284.
6) Patent Rolls 1324-1327, p. 284.
7) Calendar of Close Rolls 1323-1327, pp. 550-551; Patent Rolls 1324-1327, p. 250.
8) Patent Rolls 1324-1327, p. 286.
9) Calendar of Fine Rolls 1319-1327, pp. 375, 379, 386; Close Rolls 1323-1327, pp. 452-453; ibid. 1327-1330, p. 132, etc.
10) Seymour Phillips, Edward II (2010), p. 492.
11) Patent Rolls 1324-1327, p. 328.
12) Patent Rolls 1327-1330, pp. 10, 20, 70, 73.