Is It Okay to Harm People to Increase the Pleasure of Others?

If it was like an emergency situation where we didn't have a moment to think, I think it would be intuitive to take arms and say, you know, fight against the aliens. But if we had time to establish some facts about the aliens, so say, for example, it turns out they were very, very similar to us in terms of sentience ability to feel pleasure and pain. You might, for deontological reasons, simply oppose that attack but then you're moving away from what I take to be effective altruism.