Not wanting to conquer and only fighting when provoked is fine- it's the fact that they'll just wipe out a civilization if they ARE provoked. Massive war that economically cripples them? That's one thing. You're basically describing an ethnic cleansing on an interplanetary scale, which I don't think ANY other civilization would stand for.
As far as it being an assassination attempt on a crown prince, then depending on alliances and loyalty divisions I could see that being the spark that set off an intergalactic version of world war one, but there would be supporters on both sides and it's wouldn't just be "yeah they got wiped out". Any prolonged battle on that scale would be economically devastating for all parties involved- also, I'll remind you that with assassination attempts against politically important figures, especially ones part of one of the larger powers in the region, those have historically been far more likely to be a lone agent / splinter group, or else the work of someone who fancies themselves a "freedom fighter" of some sort and are from the same nationality / region as their target.
If this was used as the catalyst for an all-out intergalactic war, then that's fine. As a reason for why a civilization was wiped out? Still not a sufficient explanation. The only way that the galactic community would just stand aside and let them wipe out a civilization that had been on par with the rest of them was if they were doing something that was a clear and immediate threat to ALL of the other galactic civilizations, not just a threat to the governing body of one of them.
As far as it being an assassination attempt on a crown prince, then depending on alliances and loyalty divisions I could see that being the spark that set off an intergalactic version of world war one, but there would be supporters on both sides and it's wouldn't just be "yeah they got wiped out". Any prolonged battle on that scale would be economically devastating for all parties involved- also, I'll remind you that with assassination attempts against politically important figures, especially ones part of one of the larger powers in the region, those have historically been far more likely to be a lone agent / splinter group, or else the work of someone who fancies themselves a "freedom fighter" of some sort and are from the same nationality / region as their target.
If this was used as the catalyst for an all-out intergalactic war, then that's fine. As a reason for why a civilization was wiped out? Still not a sufficient explanation. The only way that the galactic community would just stand aside and let them wipe out a civilization that had been on par with the rest of them was if they were doing something that was a clear and immediate threat to ALL of the other galactic civilizations, not just a threat to the governing body of one of them.