When mulling over the particulars of any sort of magic, it's always wise to consider the pros and cons. By factoring in both sides of the coin and weighing the benefits and the drawbacks, you can come to a decision worth acting on (or, potentially, discarding a plan of action should the risks prove to be too great).
There's a phrase I've heard time & time again when learning from spirits: "what can harm can also heal". Logically, what can heal can also harm, so the equivocation goes both ways. This night not seem terribly important, but not considering this fact is a good way to get yourself in hot water.
When dealing with any magic, you're channeling forces that potentially may be outside of your control or aptitude. As with anything you eat, anything you draw through yourself changes you all the same. That's why those exercises I wrote about previously are so damned crucial.
Knowing this, consider what you're doing when you cultivate ill-intentioned energy. If you are what you eat or channel, the reality is tha tyou're basically putting liquid decay in your spiritual, emotional, and physical bodies. That malice will wear on you, and most folks aren't able to sort themselves out very easily after working with such things. That's why demonic magic, vampirism, blood magic, or other sorts of baneful work tends to cause problems on both ends of the equation.
To make matters worse, its addictive. If you're emotionally invested in doing the deed, you're going to obsess, focus on, and bathe in that muck. As Buddha once said, holding on to anger to hurt another is like carrying a hot coal for the same reason. Bear this in mind, as it will mess with you.
How much will depend on how good you are at letting go and cleansing. Some things will never wash out of your soul - that may sound melodramatic, but its true nonetheless. Keep it up long enough and you'll require the services of a sin eater like myself to begin to feel better. Sometimes its worth it, sometimes its not - up to you to make the decision. Remember, this isn't an ethical decision, so much as one that has everything to do with your well-being and long-term survival.
I've seen plenty go from magically predating upon others to harming themselves. Many develop psych issues that seriously hamper their ability to be happy or "normal".