If by justified we mean, does Tess deserve to be hanged for killing Alec, I would say no.
The subtitle of the novel, which causes controversy at the time, is "A Pure Woman," and I agree that Tess was a pure woman, more sinned against than sinning. That said, the issue is complicated. Alec ruined Tess's life when she was too young and innocent to know how to protect herself. He raped and impregnated her when she was 15, leaving her a shamed, fallen woman in their culture. Later, when Angel, her husband, comes back from Brazil, Alec's presence as her lover threatens to ruin Tess's life a second time. One can understand how the anger against him welled up inside her.
On the other hand, as Alec says, people like he and Tess "pay to the uttermost farthing" for what they have done, and this is true. Both pay with their lives, but Angel Clare, who hurt Tess even more profoundly than Alec with his hypocrisy and double standard, rejecting her for having a sexual past when he had one too, gets away with his misdeeds scot free, and in fact, watches Tess hang. It seems unfair that both the lower class Tess and the lower class Alec have to pay such a high price while the middle class Angel is allowed to go on living as if he is a good and upright person.
I would find it easier to justify Tess's execution if Angel also had to pay a legal price for what his narrow-minded sanctimoniousness did to Tess. However, since he paid nothing, I believe Tess's death was unjustified.