Who can believe the conclusion of Tess?

Many great things about "Tess of the D'Urbervilles" but the ending is not one of them. Can anyone really accept that Tess would plunge a knife through Alec's heart? No, she's not a killer - we had seen her tenderly break the necks of the wounded partridges to spare them their suffering, and she's a farm girl and used to death, but she would not kill Alec. If Tess were a French novel, you could imagine that when self-righteous husband Angel would at last return for her he would find her a changed woman - she had run of with the louche Alec and had become accustomed to the jewels and fine clothing and the accouterments of wealth. But it's not, it's an incredibly English novel - Tess is unchanged, still the dairymaid, and wants to live the simple life with Angel - but she sends him away ashamed of what she has become and how far she has fallen - all of that is credible and painful, but not that she would stab Alec to death and take off in pursuit of Angel (and overtake him, how improbable is that). Yet she does, and they wander the countryside, at last spending a night together in a vacant mansion, which Thomas Hardy describes very effectively and it's a nice echo of their wedding night in the dank mill cottage when their lives fell apart. Then they wander, vaguely heading for some Northern port to escape when in a most unlikely and stagey scene they find themselves at Stonehenge where they are captured - use of national monuments for melodrama okay in Hitchcock but out of place here, Hardy would have done better to have them overtaken in some remote valley, where most of the novel takes place. Some amazing and moving scenes in this unique and strange novel. But the conclusion? Hardy was able to break away from Victorian convention and manage a rueful, even tragic conclusion - but he could have toned down the melodrama. There, he falls prey to the conventions of his age. Gimme rewrite!