I'm open to new evidence if it comes out, but if the entirety of the new context is him doing bad things days before the shooting, I still don't think it meets the definition of self-defense. Should have been arrested earlier, but that's not enough to justify lethal force on its own.
What I'm arguing is that he posed a serious threat to officers on scene. His previous behavior, which was apparently unprovoked aside from the mere fact of ICE being present, painted a picture of him as being an unstable person who would violently escalate a situation at the drop of a hat, and the outcome of his last run-in with ICE would certainly give him more reason to bear a murderous grudge than he had before. This is what distinguished him from any other protester who happened to have a gun on their person.
All of this, coupled with the fact that he evidently didn't bring a gun on him the last time (since he was tackled to the ground and probably frisked) but he did this time, would have suggested to any reasonable officer that he intended to try to kill them, either from the onset or upon something happening that he could choose to interpret as further provocation (as being tackled again would've accomplished).
This being the case, the only criticism I can see holding water as to the officers' conduct that day centers around this question: should they reasonably have believed that the level of risk to themselves had been reduced to the extent that would no longer justify use of lethal force?
Like, sure, if an officer walks past a 4 year old wearing a thick overcoat the chance of some nefarious actor having strapped a bomb underneath isn't technically 0%, but we'd agree that a cop randomly shooting a toddler wouldn't be justified just because he happened to run this thought experiment in his head. So officers are obliged to accept some degree of risk to their persons as a hazard of the job before escalating.
But said obligation has its limits. At some point it becomes justified to defend themselves by killing someone else. The question is whether that threshold was met, both in terms of what they knew and what they didn't know in the heat of the moment, when they opened fire on Pretti. Did the officer who opened fire know he'd been disarmed, that he was unable to reach for the confiscated gun, that he wasn't on the verge of overpowering the officers and doing so, that he didn't have a concealed second gun, or that he was unable to reach for said gun even if it existed?