Of course they could. If this is about human endurance, of course someone could be strong enough to carry a hatred all their life, we never know the limit and capacity of each people in this world. But of course it is not healthy. They would eventually get sick be it within their body or mental health. Hatred is a negative emotions that needs to be released, need to go and not staying for long. Letting them sit inside you is not going to bring you good thing. However I know it is hard to let go of hatred, I'm also the one who hold grudges and so I hope whoever in the same situation will find their peace as soon as possible. Good luck, us.