Zen practice is like sharpening a blade, you can use the sharp blade to prepare food and put love in it, or you can use the blade to kill someone innocent. Blade sharpness does not have much to do with the intention of how to use it. Granted, it is probably much more difficult to reach Zen with a hateful, angry soul, but not impossible.
That is why, I believe, the Zen practice made so much sense for samurai. A samurai is a killer, and there is nothing more horrifying than looking into someone's eye and cut his head. However, they could do it without completely loosing their mind, and ultimately with some form of grace and integrity.
Zen is more like a state of stillness, you can have a day full of anger, resentment, self-hatred, and then go swim and reach a stillness moment.