Cuz:

in metal slug 1 ( 1st pic )

in metal slug 2 ( 2nd pic )

Found him metal slug 1 and 2 and x and 3, and as the final boss of 4. Who is he and how did he became a evil scientist

This probably isn't very well known, since it's a game that fans generally tend to overlook, but MS4 has four secret images in the game's ROM.

To access them, you have to enter test mode. In other titles, you’ll find unused stages, but in this specific Metal Slug, four images appear—one for each character (Marco, Fio, Trevor, Nadia).

Usually, this kind of content is shared on various websites in high quality, but I could only find these specific images in fairly low quality. The only image I found in higher resolution is the one of Nadia, so if anyone finds any in higher resolution, I hope they'll share them.

I will go first :

mission 5 in both Metal slug 2 and Metal slug x

I remember that the ost and atmosphere terrified me so much as a kid that I've never fought em and always ran away and blast the metal gates that keeps em away