I agree that Roth would continue to be a threat to Mike ,and for that reason,he had to go. Fredo could have been spared,but Mike was too weak to let him go. Had he been a truly worthy Don,he would have cut Fredo loose to fend for himself,but he was so worried about his bullshit image that he killed him. When Pentangeli met with Tom,we got a clue as to why he killed himself. He says "but their Families were taken care of ". Implying that by his suicide,his Wife and kids would be provided for. Otherwise,they would starve while he rotted away in prison till he died.