Mmm, I have to admit I rather find Robespierre more difficult to understand than Saint-Just, but I believe this is just a matter of personality, compatibility etc. Though I know what you want to say.
I think your intuition of his "power obsession" is a close explaination, but not in the sense commonly conceived - because in Saint-Just's case "power" shouldn't be meant in a strict political sense, with a merely "earthly" connotation. A wider range of meanings for the concept should be assumed. The philosophic cathegory of the nietzchean "Uebermensch" and the religious-antropologic archetype of the "dionisiac" would help us rather well. Of course this doesn't mean Saint-Just was some kind of an obsessed "Superman" (OMG !), but he strongly felt pushed to go beyond himself and he was eager of the intense sensations this pursue gives.
I have read very much about him, but I still believe that the keenest descriptions of his personality are that of Albert Camus and André Malraux, who are antropology-inclined philosophers, plus men of action and not professional historians. Saint-Just intensely looked for power as his own's mean to transcend his Ego, so this power had to be something embracing all the aspects of human life: power as a whole expression of vitality. Because of this his behaviour (and his personality in general too) could seem so contradictory and mutable... I guess Antoine was drunk, drunk of himself - or this is what he longed for, at least :)