I can understand a bit of how you are feeling .
many years ago , we 'rescued ' a wolfhound cross - friendly girl, obedient, - but a hunter . That was fine - we live in the bush - plenty of rabbits , and the odd pig. fast forward to the awful day she killed one of our bitches who was in whelp
She was happy to see her humans come home ...but . For everyone's peace of mind, she was euthanased. It is just not worth the heartache /the 'what ifs/the constant preventative measures...
Your Poor Sam his personality will change from being attacked. YOU will change from having to try & manage Rebel's behaviour. REBEL is NOT a happy and comfortable dog .
You love him? He is lucky. he gets to be respected and sent on "the long sleep" by people who DO love him and want there to be no more mistakes- no more pain.
In my mind there is only this one plan of action- sorry.