Ok, I have 700+ units. Is Cruelty that worth it or do I save for a 4-star?
How are you spending your time in the game? Are you playing arenas enough to earn 3* heroes or better? Or playing story quests mostly, and maybe picking up some milestones in arenas but not really building up your stable of heroes from arena rewards other than milestone PHCs yet?
If you are mostly doing quests I might save for the 4*. Having one can let you go a lot further. If you're spending most of your time in arenas, then one 4* won't matter that much and I'd probably go for Cruelty.
As for how helpful it is, when you max it, every one of those hits where the damage is in yellow, you'll do an extra 50% damage. Just watch as you fight and see how much that would be.
But I would put points into Dexterity before Cruelty if you haven't.