they don't really alter pitch... not really. obviously there are pitches that make up the sound but that's not really related to what you're talking about. If you know the death cover of painkiller, that's about the best thing there is. mikael or any other death metal vocalist isn't adjusting the pitch per se, just tightening his throat to reach different levels that are sounds.
that description is a little vague, but think of this. from just death metal vocals, there is no identifiable tonal pitch. there just isn't. if mikael growls something without changing any level, you can't say "that's an F#" because it isn't. instead, as it is more of a noise and not a straight forward pitch (all tembre, no melody), there are lots of small pitches going on (very small) and they change generally in some direction to make it generally a higher sound, but still not creating a definite pitch.
painkiller chuck schulinder or some children of bodom is about the closest it gets, and you can tell the pitches they're singing. it's kind of like adding distortion to guitars, if you do absolutely ridiculous amounts, the played pitch will be pretty much unidentifiable, but with death vocals, there was never really a pitch to begin with (assuming you're not 'turning down the distortion' and trying to make pitch audible like in said examples).
