Deadman Wonderland, Kiseijuu: Sei no Kakuritsu Recommendations
In both shows, the main character gains gruesome superpowers that he doesn't want, and said powers end up changing their entire lives.
Similar to Shinichi Izumi, Igarashi Ganta comes to possess new supernatural gifts after the mysterious murder of his classmates. Both characters end up in a life or death struggle against others with similar supernatural abilities and contemplate the morality of the actions of those around them as well as their own. Both Parasyte and Deadman Wonderland discuss the overall themes of humanity, power / control, and whether the ends justify the means. Similarly, both shows have an element of the supernatural and center on gore / death as a plot component. Lastly, Parasyte and Deadman Wonderland rely heavily on character development and learning about personal
If you're looking for a dark show with plenty of action both of these fit the bill perfectly.
Both anime's have a very similar sort of powers, both of which are forced on people and they have to live with it. Well that is one boy named Shinichi Izumi who came in contact with a parasite but was able to stop it taking over his brain. Now they must co-exist and the comedy is very well done, taking the p*** out of most cliches and anime relationships. This is one of those anime's that get better as they go on with a great ending that leaves no questions un-answered.
If you like "Parasyte" you will like too Deadman, trust me!
The main characters are placed in a situation beyond their control, yet this makes them both fleshed out. These anime can also get pretty bloody and violent at times.