I'm inclined to be optimistic. Nature has been experimenting with viruses and self-replicating machines for a while now, and has yet to come up with a design that cannot be countered. Bombs are capable of causing mass destruction, but we're still a long way off from having a doomsday device as cheap as a toaster.

I also suspect that, given a century or two, we'll have discovered ways of spreading the risk of catastrophic events. Perhaps by regularly downloading our consciousness and distributing backups across the solar system, perhaps even a few sent to neighbouring stars: "Alpha Centuri Backups - Just In Case Some Jackass Blows Up The Sun".