Some of the best features of people are the result of competitive evolution -- it's not just our worst features that come from the struggle for survival.

The iguana's war on algae is definitely hell for the algae. Is not killing other animals such a great distinction, in the grand scheme of things?

If all the animals were vegetarian and weren't getting preyed on, most of them would die of starvation. (Most birds, rabbits, &c die early -- it's not like people where surviving to adulthood is the norm.) The animals that killed the other animals to have more vegetables to eat would have more offspring and be more likely to survive. Killing others not to eat them but to eat their stuff -- that's war.

The iguana situation is one where there's just not much evolutionary pressure. It doesn't generalize.