Eh the second you step beyond the gate towards the grove Astarion tells you we'd be better of leaving immediately because the goblins are coming. So without some metagaming involved I would skip the grove entirely.
Also: "killing is fun" = murder hobo = chaotic stupid again.
Following the advice of the Chaotic Evil party member is such a good idea if you don't want to take chaotic stupid choices. /s
The "killing is fun" is an option at the party with Astarion too...
I don't remember all the dialogue options everywhere in the game, but there is plenty to say you did things for your own selfishness even when helping the grove.
Sticking around in the Druid grove to help whatever is going on is a good option. Trying an evil playthrough you wouldn't be interested other than for the reward / loot.
Playing evil also doesn't mean you have to join the absolute, but it definitely means not going around helping everyone in need without any reward.
Say you take the quest from the fake paladins, you ask immediately for a reward and he tells you of his uber magic sword, then the next thought would be 'Why not just kill him and take the sword?' for an evil roleplay.
Helping people because its the right thing to do = good.
Helping for payment / personal gain = neutral.
Helping only because you can't see an illegitimate way of gaining the reward sooner = evil.
Especially if you are playing an evil cleric (or paladin at release). Corrupting good paladins is more fun than just killing them and taking their loot.