Is it legal for me to construct a pure skaven neutral deck? Seem to be quite powerful for deathmaster.

it is what lots of players do : pack all skaven cards in their deck

all you need is a capital of one faction. But i guess one or another orc card does work very well with skaven...

be reminded you can only have 3 of any-one-card in your deck, so if you like decks of around a dozen units, half of which are heroes, of which each hero can only be on the playfield one at a time... doesn't really work.

A Deck of 50 card of skaven card and some neutral card. If i can get deathmaster out in the 2nd or 3rd turn. I can kill of one of your unit once per turn.

High chance i can get a deathmaster in my starting hand. I can go for a mulligan for a 2nd try.

lots of decks kill more than one unit per turn.

Deathmaster is strong of course but quite fragile. and he can definitely kills more than one unit a turn ^^