This is AGP who conducted the massacre and they are still there to do the same. They formed allegiance with BJP now and got out of ULFA long since. Current ULFA is not Bengali Haters. Infact they protected Bengali muslim after Gujrat and Babri massacre.
Nellie massacre was perpetrated by AASU (All Assam Student Union). They later fought election as AGP (Asom Gana Parishad) while a faction joined ULFA.
The Assam movement in the late 70s was almost entirely based on the issue of infiltration from BD. The allegation was that the GoI was way too laid back to check the huge influx of such illegal immigrants from BD, which skewed the demographics of Assam against the indigenous population, and poached on their jobs. This spilled, into anti-non-Assamese movement (particularly anti-Bengali), but not to the same degree as Lachit Sena of the 60s.
While AASU/AGP came out of their anti-non-Assamese mantle, because it was clearly turning out to be counterproductive for their political aspirations, ULFA stepped in. Many of the sympathizers of this anti-non-Assamese movement, within AASU, gladly joined ULFA.
ULFA was formed for solving the exact same problems that AASU attempted to address - illegal immigration. While AASU softened up and sought peaceful means, ULFA became secessionist.
Irony is that since ULFA became secessionist, they had to hide out in neighbouring countries and BD was more than willing to give refuge to them. This meant that they had to completely abandon their anti-illegal-immigration stance the very issue that saw the start of Assam movement. On the other hand because of New Delhis extremely hardened stand, they had to abandon their hardcore anti-non-Assamese stance (although once in a while they would massacre the Biharis)
So basically todays ULFA doesnt really have an issue that connects with Assamese. They have also failed to explain how an independent Assam would be better off than it is today. No wonder they have lost their appeal among Assamese.
ULFA is now
na ghar ka, na ghat ka.
You people can save that joy for another day.