I know. I wasn't even sure why I did it -- I just saw him dying and instinctively cast a heal on him...then afterwards was like, DOH!Casting a heal on a player in combat puts you into combat (to prevent exploiting). The reason you drew aggro is because the Paladin had a very low threat value (something like 1), but your heal had a much higher threat value (depending on the size of the heal, talents, etc).
I solo'ed one up to level...8.Btw, warlocks are one of the most complicated classes to play, at least if you're grouping/raiding. They are pretty easy for soloing/pvping.
But I did play my hunter on teams and that had its own problems in terms of pet management. Solo is good, teaming demands more finesse.