The problem with running around and killing cattle is that you have to get there first. If you don�t then you might have to go without food from the meat and clothing from the hide. If you are going around killing cattle willy-nilly (i.e. beating everyone else to the game) then it is likely that many of them will start to want to kill you.
Also, how do you catch and kill the animal in the first place, you would either need to develop the skills to make your own hunting weapons, or strangle/clobber with a rock someone who already possesses them. What then? The possibility of someone else hitting you with a rock or strangling you to get them from you?
Thus it would transpire in order to survive you would need a skill that is transferable or relatively unique to your own particular environment. No good running around naked if you live in the bleak cold north. So you might require clothing. This might be down south, where there is already an abundance of people chopping up animals and using the skins themselves.
What choice then? Do you find someone who can make weapons go down there kill them all and take their animals? Fine for winter one, but after that? Thus it would seem the necessity of trade is evolved. Much better to sell your skills/goods to the south to ensure a regular supply, possibly even bridge the trade gap between the south and those even further north, near the sea � they might be able to provide whale or seal blubber and meat, essential for the harshest winters, which would not normally be accessible to you. They may have a very good way of hunting them, that it would take years for you to learn.
Admittedly tribal warfare has existed for an age, but in parallel, trade has existed for over 150,000 years. Yes money/cash is not essential to survive, but you need to be able to trade something in return for survival, even if that is the services of a marauding barbarian.