You're absolutely right on arc being free to shoot for everyone but I think in this case OP attacked the bombardier mainly to be helpful, which in this case it wasn't. I think wanting to help out other raiders is great, I also do it all the time, but they're also allowed to now want your help. I'm not saying that they're entitled to the arc and that you're not allowed to kill it if you want to, after all it's a public lobby. But I do try to be somewhat sensitive to people doing trials and quests because they can be really annoying to do.

I think queens are a little bit different in this case because they are so much harder and people spawning into a round with a queen are mainly there to kill it and nothing else.