I agree with what your father sees as terrible violence in the Old Testament, so hopefully I can be a sounding board as to some of the advice others have been giving you.
@(name removed by moderator) recommended having your dad to read the Old Testament in light of the New Testament. To someone who doesn’t believe that method doesn’t negate the violence in the OT. It just makes the doubter wonder why God the Father seems utterly capable of any kind of compassion that his son seems to show. If anything it would push your father into Marcionism.
@Aquinas11 said that God never encouraged or initiated these things. In Exodus 34 God tells Moses “I will drive out nations before you and enlarge your territory”. Regarding the plagues in Egypt not only did God harden Pharaoh’s heart to prevent him from releasing the Hebrews, but since the fault lied with Pharaoh then justice should have been applied just to him. There’s no reason to set a series of plagues which would kill many. God initiated having a man killed for picking up sticks on the Sabbath. God set forth a detailed set of instructions as to how his people could obtain and abuse slaves, even though they were in the desert at the time and had spent over 400 years prior as slaves themselves.
@Zach said God gave many warnings about the punishments that he handed down, but much of that is highly disproportionate to what was done. A good analogy would be a mugging victim getting shot because he took too long handing over his wallet. Sure, the victim was warned, but it certainly wasn’t warranted. We can look at the story with the census where a monetary tribute wasn’t given as prescribed which caused thousands to die.
Finally, @lil_user says we are not in a position to judge God’s actions. I think we certainly are. Your father absolutely does. To try and brush it off by saying God’s morality is not our own or we simply don’t understand is not an explanation but a way to excuse great evil.