Hi, I usually play Lulu support, it's very difficult to determine whether I should damage/kill a champion.
Constantly I see missed kills or raging players, could anyone with support experience help?

Honestly I play both roles a bit and the way I always see it is this... It's better to have a support that knows when to get aggro and maybe picks up a kill by chance, than to have one that doesn't know when to go in and causes teammates to be in fights that don't benefit them.

As long as your carry stays alive and is doing better than the enemy carry, then your lane is winning. That's the thing people seem to forget. If you accidentally take a kill from your carry it isn't optimal sure, but if your carry lives and the other carry loses xp and farm because he died, then that's great.

Take it whenever there's real doubt about it escaping or someone dying. If you're reasonably sure the kill is secure, try not to. Accidents will happen sometimes, but you don't always know if they had an escape about to come off of CD, so just roll with it.

As Lulu I try to avoid it, but I'll definitely go for it if we're not directly on top of that champ, or if the champ can take the heat and still burst us down, like Mundo.

Nothing is more satisfying than an enemy carry that thinks he can nail you, because your carry is in town and then you Q ->W -> E -> R -> Q + autoattack in their face and they realize you're going to get a kill. It's not like it happens often and mostly not during endgame, but early lane phase, if my carry takes too much heat; dang, I've made so many kills people started calling me solo bottom Lulu.

Just follow this mantra: "A KS is a Kill Secured".

try not to last hit if its secure but if its iffy just take it, a fed support is not a bad thing.
support items tend to help out the whole team so its not lost cash in the end.

plus put the sucker on cooldown.

The only time I try to AVOID kills as support is if its blatantly obvious the carry will get it, and expect the same from my support...assist gold works too, and as most adcs aren't"beast mode" till end game either its better to get it than risk losing it...

There are more benefits than 125 gold difference between kill and assist with only 2 people on it...missed exp and farm to name a few...

Besides...supports need gold too...kinda like "fat girls need love too"....be friendly with fat chicks, chances are, she has a hot ass friend with low self esteem, who hangs out with her to make herself feel better....easy target...likewise...supports with gold = more wards which makes you a more difficult target....

By "blatant" I mean "away from towers, in the river, after ive thrown exhaust, the carry has thrown ignite and cait ultimate is locked on the enemy"

Everyone here has suggested what I would also agree with. Take the kill unless its just so obvious that the enemy has no chance of getting away. People will always complain about the support getting kills but like everyone said here it is not a bad thing, and a good support not only will buy gear to help the whole team but if fed enough they can even start to pump out dmg themselves provided they have map control and all that covered.

I actually recently had a team tell me when I had gotten a few too many kills as a support, which some were just from teamfighting and sometimes your toon just auto attacks and you get lucky. But they flat out told me I was going to make our entire team lose the game because I was getting kills. It just isn't true. The big thing to remember in the end, like I've seen said so many times on the forums here. Is kills don't win the game. They may help sure, but objectives is how you win the game. If that enemy is dead no matter who killed them and you get to take down a tower because of it, your one step closer to victory.

I go for the kill anytime it looks like the target will escape. Sometimes this is hard to judge but ensuring the kill is better than losing it and the carry will still be getting some gold out of it.