To hate the man, you would have to hate every single weapons designer ever. And to hate every single weapons designer ever would be to hate any need for weapons. Unless you are a COMPLETE pacifist, you recognize that there are some legitimate uses for weapons (for example, the police using a Glock Pistol to shoot an armed robber). Therefore, you MUST logically recognize that SOMEONE has to design those guns and that it is legitimate for them to design such a weapon. To completely hate a weapons designer, you'd have to completely swear off any State from using force for any means whatsoever.
Yes, the AK-47 and its many variants have been used in many conflicts. I can guarantee you 100% that if the AK-47 had never been invented, they'd be using something else, but they wouldn't have stopped fighting. They would have used a different weapon - someone else would have stepped in to fill the gap.
He wasn't designing it to be used by terrorist groups or child-soldiers. He designed it for what I feel is a LEGITIMATE PURPOSE - for the military of his country. The only way you could say that it was a bad thing for him to design that weapon is if you take the COMPLETE pacifist mind-set of never using force for any purpose whatsoever.
If you are a complete pacifist, I wish you luck. You're gonna need it.
(P.S If you are a COMPLETE pacifist, you also have to swear off on calling the police to help you - it's not enough for you to not use force, you can't let anyone else use force on your behalf - seriously, if you are a complete pacifist, living in any nation with a police force that will protect you using force is hypocrisy - go live in the wilderness, advertise that you will not accept police assistance. We'll see how long you last).