As you guys probably know, the game tells you on some loading screens you can get an ending that is less dark if you minimize your kills.

Thus far, I've limited kills to the assassins who are sent to hunt my character down... hopefully that's good enough.

Honestly, the actual gameplay seems a lot more fun when you are actually fighting and killing instead of sneaking around doing non-lethal takedowns all the time to get the happier ending, with some exceptions (Like the Lady Boyle mission). I am feeling like I'd like to just blast away with the upgraded pistol and crossbow... at least now and then. But... I want the better ending. =/

The point of the good ending rather than going full stealth is going non-lethal or as non-lethal as possible. There have been multiple times during which I have been zipping about alerting guards for example to save someone (there are multiple instances of people you can save from bad situations usually because of the guards). Of course I always did full non-lethal (because I enjoyed the novelty of just going about without killing anyone) but it's perfectly possible to kill some people every now and then. Stealth though, I was never really good at it so there were multiple times I blinked away from alarmed guards or just raised some hell by saving some innocents from dirty guards.
Also the heart can tell you how killing someone can influence chaos levels, basically if the heart tells you someone is a serial killer killing them won't really raise chaos too much but a more morally average or upstanding person will jolt the chaos level up more.

The sequel does allow for a less stealthy non-lethal playthrough, you can for example stun and put into a chokehold people during a duel and to non-lethal drop-takedowns.

I found out apparently the game is pretty forgiving about kills in terms of getting the good ending. As long as your overall chaos level is "low" by the time you end the game, you're still on track for the more desirable ending.