I love this novel. I subscribe to the whole d'Urberville blood being part of the reason for stabbing Alec- this is after all Tess of The D'Urbervilles.
A lot of fatal mistakes are made in Hardy's novels. I don't think Tess killed Alec in cold blood but as a desperate impulsive mistake. Perhaps there was an element of self-loathing as well- Alec represented that stain on her purity that prevented her being with Angel.
I don't find Alec unsympathetic. I think the key to this is his mother, who significantly is blind, preferring her birds over her son. As Alec has not grown up knowing love, he persues lust instead. He is incapable of love because he has never been shown any. The only point where he might have shown love towards Tess is after he has destroyed her.