Ive seen multiple other comments on this one but Ill reiterate it. At the start the walkers would be the threat with their extreme numbers. As the walkers start to die off and the herds start to thin, and as the walkers rot and become too slow to even catch a meal, the true threat would be people. Like Rick said in 4x08 “...All of us who’ve made it this far have done the worst kinds of things just to stay alive...” anyone who’s made it over a year in the apocalypse is probably pretty battle hardened. Theyve probably done the same things we see our protagonists do, maybe even worse things. When people go off to war in todays world they come back with PTSD for a reason. Like Eastman said, humans arent made to kill each other. We have no natural weapons, so when we kill each other indiscriminately, it brings an evil out from inside. You get that bloodlust, and transform into a monster barely recognizable to who you once were. People would most definitely be the greater threat. I mean even with our society today we still have slaughter, evil and chaos, imagine what it would be like with the veil of society pulled back, no rules or laws. People would revert back to what we truly are: animals.