For a farmer, his farm is sacred and no one should touch except for him since that is his livelihood. Without his farm, there is no way that he can earn a living. Seeing his farm get destroyed by a catastrophe is already hard for a farmer, but seeing it being destroyed by other people make him angrier. At least, he can do something about it when other people want his farm destroyed. That is the case with your farm as someone want to buy it, but you don't want to sell it. The one, who wanted your farm want to get it, so he hired some goons to make you decide to leave the farm. But, you are a top lad and you don't want to leave your farm. You would fight for your farm even if it means killing someone.
In this game, you need to kill people in order to save your farm. These are hostiles since they want to destroy your farm. Actually, most of them want to hurt you, so it is just right to defend yourself. Armed with only a truck, you need to run over the enemies to get rid of them.