Your reasoning on guns was correct. Remember how almost every year we worry about what kind of influenza we'll get? We all worry about a repeat of the 1918 flu, which is quite likely on any given year.
Remember how antibiotic resistance keeps getting worse and worse (or better and better from the bacteria's perspective)?
HIV originated in nature, probably somewhere not too far from where Ebola is natural brewing now.
Our number one threat is mother nature. Her and other humans living close to domestic and wild animals around the world and air lines.
At any given time we might have to cope with an 1918 like flu.
Custom tailoring a virus to include one people but not other is going to be extremely hard. We are all very closely related. Human conflict tends to be between neighbors, and for example the people most genetically similar to the Israelis are the Palestinians. So the probability of a custom tailored virus is far, far, FAR lower than the probability of a natural don't-give-a-crap-who-it-kills virus.