For something completely different, Crazy religions fanatic.
We already have Westboro Baptist Church that performs baptisms and conversions of the dead against their will, but at least from a distance, so for horror take it another step.
Have a religious organisation unhappy that someone was burred with Egyptian rituals instead of Christian ones. This is ...
The person wants the valuables inside.
He purely want's to desecrate it because his is a highly religious person of a other faith who thinks that Pharoahs who functioned as a bridge between the (in his eyes false) gods and the people is heresy.
He believes he is the descendant of the pharaoh and therefor the rightful ruler and hopes to find some evidence ...
Wealth. Digging up tombs to loot them is a time-honoured human activity.
In trying to explain any kind of stupid human behaviour, you can always count on the old reliables: ego, sex, wealth, and religion. People will do all sorts of dumbass things for one or more of those reasons.