One of the biggest questions about the current series is how much the previous IDW Megatron influences the current Megatron.
The former makes it a decision based on a known threat, but the latter makes it a decision based on paranoid speculation that seems to be based solely on the Reversionists not being Autobots, and therefore scary.
We do not know enough about the various factions to be sure if "not Autobot" is enough to make a group automatically suspect. Reversionists could be a nominally peaceful group (like the Greens) or openly willing to use violence (such as the Environmental Liberation Front).
I am inclined to think that the Autobot concerns are valid, based on actual precedent for this sort of thing.
Example: Catalonia. Catalonia wants to break away from Spain for wholly economic reasons, specifically that Catalonia is tired of carrying the dead weight that most of Spain actually is. They are receiving support from Spain's socialist party. This is counter-intuitive, as a socialist party would be more likely to try to keep an economically viable region in an otherwise failed state. But, the two groups are making common cause against another, right-wing, faction.
In this light, I can comprehend a "return to Primus" group making common cause with expansionists (such as the Ascenticons, or even the Rise). Even assuming that the Autobots are a tyrannical majority, they would have more than good enough reason to be concerned about two minority factions making common cause against them.
. After the first issue came out, this was something Dom and I somewhat debated, with Dom saying he believed Megatron was embracing the violence. The fact Megatron says he'll publicly condemn the violence in this issue just proves what I'd been saying.
Part of my argument was disputing your reasoning that Megatron was exculpated by the fact that the crowds at his rallies were no established characters.
And, there is a difference between playing to a (pre-existing) violent base and fomenting (new) violence. My guess is that Megatron is willing to take support (votes, or political agitation) from the Rise. And, at some point, he will either lose control of them, or be forced to take ownership.
I am more inclined to give Megatron the benefit of the doubt at this point though.
The fact that Ruckley is avoiding getting back to the assassination attempt in issue 2 is a clear indication that there is a twist on this front.
Case 1:
Megatron engineered the incident, in an attempt to rally support and create the impression of state (Autobot) terror against the Ascenticons. (Unlikely, and boring.)
Case 2:
The Rise is attempting to foment violence among the Megatron's Ascenticons by making it appear that the state has attempted to assassinate Megatron. Megatron and the Ascenticons believe the ruse. (Likely, and intriguing.)
Case 3:
An arm of the state (though not one of Prime's group) attempted to assassinate Megatron, creating a legitimate reason for Megatron to co-opt the Rise (and possibly other factions). (Unlikely, but intriguing.)