So I recently finished Doom 1 and Doom 2 on Steam using the keyboard.I can't really use the mouse+keyboard combo,so I used keyboard only.
Even with that,I have serious problems to avoid enemies bullets.
Now I tried Doom 1 with the 360 pad and I completed the first two levels on Ultraviolence with no death easly.
But..why?
Why I can dodge enemies attacks with a gamepad but I take ALL of them with keyboard?

I'd say it depends on your keyboard setup. If you couldn't strafe easily with your keymapping, it's no wonder you got hit a lot.

Could you even use a mouse in Doom 1+2?

Nope,with mouse I take AGES to round a corner lol

Source port... use Zdoom.

That sounds like your sensitivity was set WAY too low. Have you tried one of the modern source ports for Doom 1+2? They offer much better control options than the games did originally.

Edit: Aaaaand ninja'd!

Don't know what everbody's problem is. I mastered the default keyboard layout when I was 11 or 12, beat Doom 2 on Ultra Violence with it.

I tend to enjoy those shooters more when i not use the mouse. I've been playing them without mouse ever since they came out and i still play them so i've gotten really good at them. It's usefull to hold the sprint key 90% of the time (or use auto run). You will turn faster like that.

Mouselook always feels wrong to me (controls strange and feels like cheating) but when it works i don't mind using the mouse for looking left or right more and more, an unavoidable side effect of games using mouselook since circa quake i suppose and now all of them use it so... ;).

A bit more on topic i must admit that after playing doom en duke on the xbox 360 (I compulsivly seem to buy almost every version that comes out), i was surprised how well it indeed worked with the controller. (Had forgotten doom worked perfectly on the playstation (1) too.)

Quake was the game that switched me over to mouse+keyboard instead of just keyboard. After having my ass handed to me repeatedly in multiplayer, I had to make the switch, and I never looked back since.

The 2d hit detection makes a gamepad work fine with Doom and Doom 2. Duke Nukem 3d is where I started using the mouse for a FPS as aiming up and down with gamepad buttons was bad.

After years of playing with WASD + mouse look, it's hard to go back to where you shoot with Ctrl button, open doors with Space, and move around with directional arrows.

It's certainly not impossible, of course, it's just a more difficult scheme. In ZDoom (and most of the other ports) you can disable mouselook on the vertical axis to give a more authentic experience. This is often a good idea anyway, as looking up and down a lot in Doom is a good way to give yourself motion sickness due to how the corners of the screen distort. In some cases vertical mouselook CAN be akin to cheating, especially with stuff like the final level of Doom 2 where you can just stand on the top of the pillar and fire away without having to worry about timing. Under most circumstances though it doesn't really give any advantage above the basic auto-aim.

... it was so fucking hard to adapt to mouse look, it's weird to think that I used to play Quake with the number pad.

I somehow skipped over quake (my father bought the games for me) so the first one for me was Unreal. I lingered a long time on the older shooters. But they kept me entertained, that would not work these days ;)

Back in those days we rocket jumped through lava to school. UPHILL!