I think you maybe painting the 'west' as one entity. Once upon a time the UK really did rule the world, then America, not really the same.
As to why go to other countries to fight, because we can, and it moves most of the fight away from our shores.
Obviously any country accused today of mistreating a minority or whatever, by a western nation, is going to throw empire building and killing the natives back. Doesnt make it right, then or now. Equally countries have to deal with issues, dont recall many countries worrying about Northern Ireland, other that soviets and americans sending weapons......
The whole China thing, they arent going to take over the world, nor run it, not make everything. I've said before, once people get to 'middle class' status, they prefer not to see it blown up, nor send their sons off to fight etc. as they dont have a spare son anymore. Once the cost to make something in china is 90% of the cost in Europe, the work will come back, to mostly robotic factories, while we all serve moccafrapechonis to each other.
Also just asking, if china really wanted to expand and conquer, Philippines would be high on the list, we kind off assume you'd like our assistance in terms of the South china sea islands etc? If we took the chinese request to get lost, really they are just asking to be allowed to bully you all into surrendering anything they are interested in, to them? dont you think? of course the benefit from us, is to tie up china for 20 years, in getting hold of some island with gas/oil under it, or whatever. Really same for Tiawan, we font give a s**t about tiawan, but it ties up the commies a bit longer.