I wouldn't worry about plants. Plants actually need to have things 'kill' them for their own health. If animals and other things weren't killing them and thinning them out regularly, they would grow so much, they'd die on their own because they either can't get enough nutrients from the ground or just start rotting from the inside.

They don't feel pain, they can't think, they can't even move, and you're doing the species itself a favor by making sure it doesn't overgrow itself. If they could suffer, they would be suffering if they weren't thinned out.