No! Unethical would’ve been if Sen poached him while he was under contract, if he wasn’t under contract and was allowed to look for options then it’s not unethical. Sentinels bought him out, if they asked him to leave the team without buying him out then it would’ve been Unethical.
Wanting more money isn’t unethical. You’d understand this when you work in an organization and you want good pay and get offered one by another company, you’d take it without blinking an eye.
Also he asked his teammates to join. That means he wasn’t selfish and wanted everyone to get more money. I don’t know what’s the definition of snake in your book. But that’s sure as hell isn’t a snake