A non-violent species has to be herbivorous, since a carnivorous lifestyle requires violence against prey. The problem is that plant matter contains much less nutrients than meat, so the chance of a completely herbivorous species evolving to have a big brain is quite small.
It's true that humans today can survive as vegetarians, but only because we cook our food (to make nutrients easier extractable) and have a network of global trade to provide us with a huge diversity of plants and produce that wouldn't be available on an isolated island.
It takes intelligence to be able to utilize fire to cook food. Your species might never evolve a complex enough brain to take this nessecary step towards higher intelligence. Maybe the sweet fruits of a tropical island could provide enough nutrients, but then you're still missing the incentive to evolve.
A life without enemies and with energy-rich food available at all times would hinder evolution rather than advance it.
If a creature can sleep without fear of being killed, wake up without fear of not finding enough food to fill its stomach, be lazy without fear of any dire consequences, why should it invest energy into developing a big brain?