Ate steak with the layer of fat kind of undercooked, is it dangerous?
It is not (immediately) hazardous to your health.  I say immediately because I would not recommend eating fat or gristle en masse.

I used to marinate my steak every time I prepared it.  I have gotten away from this habit.  Marinating breaks down the integrity of steak.  This is desired if you are cooking a less tender cut.  I eat steak less frequently now, and for that reason, I choose to get ribeye or strip.  If you decide to go this route, using dry rubs in concert with a good sear, or grilling, will offer you a very tasty meal.  Also, you will be able to gauge what fat and gristle really are - tasteless with a sickly texture.

As far as why eating steak like you mentioned isn't hazardous, given your description, it was cooked at least medium.  There is a large segment of the population which chooses to eat rare steak.  This type is deep red and cold in the center.  While it's not for me, I've never heard a Surgeon General warning that says rare steak should not be eaten.