A lot of Opeth's stuff isn't too difficult, but still brilliant.

Just a couple of easy death metal songs off the top of my head are Corporal Jigsore Quandary - Carcass and Hammer Smashed Face - Cannibal Corpse (not my favourite band but it is a commonly known death metal song).

Some easy songs of other metal genres are:
Mayhem - Funeral Fog, Deathcrush, Chainsaw Gutsf*ck and Pure F*cking Armageddon
Strapping Young Lad - Consequence
Pantera - Walk, F*cking Hostile, 5 Minutes Alone... many others
Slayer - South Of Heaven, Raining Blood, Dead Skin Mask and Disiple

Your main consern with these songs would be learning to tremolo pick (not very difficult) and artificial harmonics (can be a bit tricky at first but are good to learnt because they are used quite often in metal songs)