How are you saying Serious Sam was scary? I always found it as a straight up action game with funny monsters and guns.
the monster aint gonna make you screem. they gonna make panic and make you jump out of the chair.
What monster in serious sam would do that to you? The closest would be the kamakazi screamers, but even they aren't scary since they're so funny.
But yea I agree slender man isn't really scary (at least when I played it) there is only three things that are supposed to scare you,
1, The Unknown (the dark, slenderman's face)
2. Slenderman
3. the music
The unknown really is only scary for a little bit since you then realize that him not being in your sight is quite useful, slenderman dosen't actually move, just stands there and really is just placed at random near you to make you freak out, the music is actually scary unless you're used to music like that in which case it will do nothing to you other then say "you're getting close to the end" or "slendermen is in your view"
Btw I played the greenteam version and the first one of those high graphics ones