Bratty boy - for moms of boys only

How do you get your toddler boy to become more nice and gentle?? We've tried so many different approaches but he still likes to hit and just do things he knows he's not supposed to.. We've tried ignoring it, time outs, taking things away, even sitting and talking to him to let him understand it's not nice... I am going crazy, I get so stressed out because I just don't know what to do anymore and I want him to be good! We have a kitten and he will pick it up and throw it, grab it's legs / tail and hit it.. When he's trying to be gentle and pet the kitty he is still really rough with it. Same as with other kids, I just don't understand ;( he has good manners and says please & thank you and always says I love you and loves to hug and kiss mommy and daddy, I just don't get it.. Any advice would be great. Google doesn't help lol.