Lifestealer is kind of crazy; lifesteal, slow, free bkb, initiation/escape. He doesn't have any real weaknesses right off the bat. You can easily farm ancients by level 4, and he doesn't need any special items. Pretty good hero. We like to run him with someone who can initiate so he can ride, like natures prophet, queen of pain, bounty hunter, riki, spiritbreaker, etc.
You can't really rate your games by your KDA though, if it's a low kill or short game you might have less, and if you play an initiator you will have more deaths. If you are a disabler you might not even get assists. For some roles its bad to get kills, like Elder Titan, I don't think you were the carry or a ganker, so try not to last hit the enemy heroes :P