Its nothing of the sort. He wrote letters to the president stating that the Germans were working with this technology and that their were local US scientists doing research on it as well.
The arms race then started. Einstein knew that someone was gonna make the bomb. He merely chose which side he thought would do the least harm with it. He already knew the Germans were slaughtering people left and right.

He may have personally believed that wars should not be fought, and he may have been a pacifist, but he was intelligent enough to know that his ideals wouldn't keep the rest of the world from fighting.

If he didn't write those letters then we may have lost the arms race and Germany could have possibly ruined much of the world as we know it.

Does this mean he is not a pacifist? No, it means he is a great man, because a great man can look past his ideals if it means he has a chance at saving lives (or the world for that matter).