Of course it is, but responding to a personal attack with another personal attack doesn't negate the original one, it just adds another on top. If you want to be able to have a calmly rational discussion, you need to be able to resist responding to disruptive comments in kind, otherwise it only takes a single person to derail an entire conversation and ruin it for everyone.
Being unable to engage with certain viewpoints to the point of bringing up foreign infiltrators every time you see it is a sign of brainwashing. I stated that as calmly as possible. I have received far worse replies on this forum.
What really derails the conversation is needlessly tone-policing people. People have a tendency of only doing that to opinions they disagree with; so in practice it's just another way to start a flamewar.
I commented on your tone exactly because I frequently find myself agreeing with you when other people don't. If I disagree with someone in both content and tone, I won't bother engaging, I'll just downvote, flag and move on.