For me, Anders is a terrorist. He killed innocent people in a Chantry as an attempt
to achieve independence of all mages. Given Hawke was the Champion at that point
it may have even been possible for Hawke to negotiate independence for the Kirkwall
Circle given his/her political position. He should have at least consulted Hawke first
to endeavour with talks before going ahead with such extreme methods to secure complete autonomy.
Ander's is alive and in love with my female Hawke in my Dragon Age World not because I
like him, hell I resent his actions but simply to make the story all the more interesting.
Especially given my Inquisitor will be a Templar/Chantry loyalist who much like Cassandra, does
not particularly like mages. 
That said, it may be a superficial decision if you kept him alive or not given Justice/Vengeance
is alive regardless of you're actions in DA2. It may just be the same cut scene or role (if any)
but with Anders or another person that Justice/Vengeance has taken control of if Anders is dead.
Potentially similar to whether or not you kept the Council alive in Mass Effect 1, the impact on the plot is
negligible.