Clarence, it looks like I’m getting to this late, but here’s a thought or two:
Evaluating imperialism as a bad thing seems obvious at first glance from a libertarian perspective, and I think even after careful examination it comes out as a net negative. However, if the concern is for individual liberty, it’s worth pointing out that Western imperialism did increase that in many respects by doing things like ending the slave trade and oppressive local practices such as suttee. It also increased the worldwide division of labor and improved standards of living.