In the eyes of most of those who intended to convert people they were doing them a favour since they were convinced that heaven only awaited those who followed their faith. Compare it to vegans who try to convert people to their dietary choices - they may be annoying but they're not evil.
Compare it to converting people to washing hands with soap, and to taking vaccines against deadly diseases. Both ideas require some rather elaborate mental framework which can't be trivially proven and requires study (and thus language proficiency). Both require some outwardly weird actions. Both ideas most literally save lives and stave off a lot of suffering. People actually go out and send expeditions into poorest regions of the world to bring these benefits there, at no cost to the locals, and at a significant cost to themselves.
Now imagine that instead of saving people from cholera and plague you save them from death altogether. It should feel like a huge moral duty, and like the most important thing to ever do to those unenlightened.
