In his own way, Angel's behaviour toward Tess is just as reprehensible as Alec's.
She killed Alec because - well, it's Thomas Hardy!
But thinking this throug in more detail: f you look into the contemporary background influences on this novel, it's full of materialistic, physical elements of 'love' - materiality as represented by Alec and his phony 'conversion' to try to atone for his sin, and the old neo-platonist idea of sublimation of the sexual impulse into higher cultural ends. This is symbolised by Angel Clare - note the significance of the names, the first one being self-explanatory, the second meaning 'light'. A comparative example is the last sublimation of Dante in 'Paradiso', when he quite literally sees the light in his vision of the Virgin Mary when Beatrice returns to the empyrean. Tess's ecstatic response when Angel plays the harp hints toward this idea - at this point the relationship remains unconsummated, which adds to the idea of this 'pure' sublimation (hence the subtitle is a 'pure woman', to which the Victorian audience took gratuitous offence as Tess wasn't sexually 'innocent'.
Tess is a deeply philosophical novel and Hardy was interested in contemporary philosophy. This novel is bound up with his ideas of immanent will and a force governing human interaction that is beyond our control. He wasn't an idealist, but there's a good bit of transcendentalism in there as well, I think. All very interesting in accordance with his alleged atheism and responses to issues of spiritual 'reality', or some realm beyond material surfaces.
I think this is why Alec and Angel both have to be there, performing their respective and distinct roles in the novel which represent both these possibilities. It's why Alec has to die, and it's also why Hardy doesn't let Angel off the hook for his cruelty.
Wonderful thread: thanks for posting!