Fair-enough - but choosing 'drain life' as a level-up power would be more of a dark-side choice IMO - it's not like your character knows about the end-game.
True, it was ultimately a gameplay choice that I made with defeating Malak in mind.
However, using Kyle Katarn's words - when he was already a Jedi Master (in Jedi Knight: Jedi Academy), he said something along the line of, it's not what powers do you use, but why/to what goal. By that logic, using "Dark" power of Drain Life in order to defeat Malak might be considered a-okay by Jedi standards (especially since I actually first tried to turn him back to Light side, and only fought him when that failed - oh, and actually apologised for first leading him down the "dark path" too, afterwards).
Also I'm not sure if picking powers in the first place is anything but Gameplay and Story segregation - in the movies, we don't see, say, Luke advancing in levels, and picking Force Speed, or Force Persuade power, etc.
Even if it isn't though, it could be explained without much of a stretch as some remnants/resurfacing of Revan's power/memories. Or him not being as perfect a Jedi as there are. Etc.