Is there a problem with adopting the pacifist ethos? I think pacifism, correctly considered can be a legitimate expression of one’s Christian faith.
That being said, I don’t think the sermon on the mount demands pacificism to the exclusion of a “just war” or self-defense. It does say “blessed are the peace makers” and it condemns violence and anger. However, consider a situation where a dictator is committing genocide against one or more groups of people who are unable to stand up to the dictator themselves. I don’t think Jesus is saying that we ought to let the dictator do that in order to preserve the peace. The fact of the matter is that in such a situation, the peace is already broken, and in order to make peace again, it can be necessary sometimes to make war. I think examples like this one shows that self-defense, or defense of another can be a legitimate reason to be violent, and this does not necessarily constitute a violation of the commandment.
On the other hand, one should also note that there are legitimate peaceful responses to the situation. So, violence should always be a last resort, and proportional to the threat presented and goal intended.
I hope this helped! God bless!