I think it is a stretch to argue that every missionary is motivated by 'Imperialism'. Certainly some may have been in the past, but if one sincerely accepts the tenants of a religion then it isn't surprising one has enthusiasm to spread it. I've seen plenty of evangelists for every ideology under the sun. People who think they are right will go to great lengths to spread their ideas.

Also, the end of slavery was pushed by conscientious Christian evangelists like John Newton and William Wilberforce in the UK. It is easy to argue against slavery in Christianity owing to the Imago Dei doctrine. I suspect it is less easy to argue against slavery in other religions and cultures; many of which still practice forms of it today.

Spreading your religion while separating your culture is difficult. The same way it is difficult for a migrant to fully adapt the customs of any host culture. I think the relationships between these ideas are more complex than the evil catch-all of 'Imperialism!'.