He was trying to return Damien and Vaselle home, presumably to Freehold, and also might need to go into Murgatory.
Remember at the end of Book 2 when Tizzy opened the gateway he was also Edwyrd.
Big point is that a giant demon walking around is a lot more conspicuous than an orc or human.
If he had an orc form, he would most likely have gone as an orc, at least the second time. Also, by the second time he was aware of the hullabaloo they caused the last time.
I can put something in there explaining that he’d chosen to go as human so as not to attract too much attention.