Finish the thought.

"Humans are violent creatures that bring death and destruction wherever they go so war is an inevitability... so when war inevitably is called for, you need to step up and fight for the right side"

Howl's big arc in the film is growing from a detached egotist into some that cares about others. Why do you think he lives in a moving castle? He has walls up. He is the moving castle.

The film absolutely criticises Howl for ignoring the pleas of people who need him to help. When he does participate, this is seen as character growth and a big part of his development.

Again, this is where we can see their maturity when tackling the subject comes in because you're right. Fighting in the war "makes him a monster". Just how like fighting in WWI made Porco a pig, or how Seita's living situation was unfair but the right thing to do was to bear it, or how Jiro's plane designs killed countless people. A simpler movie would have had Howl beat the monster to the cheers of everyone. Happy ending, everyone dances to Smash Mouth. But while Ghibli films accept the need for war, they never sugar coat war.

Howl was a monster for killing people, but sometimes you need to be a monster. We also, again, have that element of the war in HMC being a fake war. But they really do stress that Howl was selfish in the scenes where he was lounging in bed while people died.