"The perpetrators' motives were rational, based on America's policies and actions" is one thing (which I happen to agree with), but anytime you start indirectly justifying civilian massacres, it's a slippery slope. "America" isn't just the government but also the set of people living in the country.
Did Japan deserve the nuclear bombings, and did post-defeat Germany deserve ransacking and worse? On one hand, of course the leadership deserved severe reprisal, and their governments and forces did even worse things when taken in total, but if you're a person on the ground seeing civilians brutalized and killed, "desert" becomes a much more flawed notion.
An example for watchers of Attack on Titan, without spoiling anything: did [X] deserve [Y]?
So, I probably agree with that Twitch streamer's sentiment, but I disagree with the way they said it and I understand why Twitch suspended them.
Why do you say that? I would definitely file that under controversial political opinion. That opinion isn't calling for another one, or saying that the individuals killed deserved it.
Especially because he's saying it out of disappointment, not anger.