was ultra seven x
 

really violent and horror stuff like seinen manga or not?

Not quite, X was targeted at adults so it did have a darker tone, but it wasn't overly violent, there's only one episode that I would truly consider has horror.

There's also the fact that the battles in the show are incredibly short, so generally all that happens is that the monster does a few attacks and Seven finishes it with a single attack. Not particulary violent.