There're a couple of scenarios under which Torm can take Tyr's place and still stay true to his ethos. Take the inevitable loss of worshippers that Tyr is bound to see as a result of this. Remember, Helm lost a ton of worshippers for far less (Maztica, which he didn't even order, and the Time of Troubles, where he was just following his ethos), whereas Tyr's actions were horrid and petty. So he's bound to suffer losses that would make the losses the Helmite Church suffered pale in comparison. That kind of loss could be enough to dip him into Intermediate God status (After all, who can trust Tyr's judgment anymore? Especially after misjudging his own friend so horribly that he killed him).
Then there's the enmity that'll be directed at his church from numerous sources, including natural allies. Ilmater and Siamorphe, formerly his servants, abandoned the House of the Triad. And their worshippers probably look at Tyr with antipathy now. Tymora's worshippers are doubtlessly less then thrilled with him, either. And of course, Ex-Helm worshippers must despise Tyr. And then there're those who worship the Triad as a whole. With the loss of Ilmater, there's no more Triad. And it isn't Ilmater who's going to be catching the blame from the likes of the Triadic Knights.
So not only has Tyr weakened his own position, but he's also burned half-a-dozen bridges, mostly with people who were and should be his allies. So one can see several results spinning off from that.
1) Tyr realizes that he's thrown the Balance all out of whack, and that the dislike that people have for him personally will keep it damaged. So being the staunch supporter of maintaining the Balance that he is, he gives up his position, passing it to Torm, and moves on to other worlds.
2) Ao punishes Tyr for doing a poor job. After all, Tyr's the one and only LG Greater God. It wouldn't do anything for the Balance if he were to drop down to Intermediate God status, or for him to be fracturing the Lawful side of the Balance when he can be replaced by someone who isn't so divisive.
There's also having Tyr simply leaving Faerun in shame and guilt over what he did to his friend Helm. Just a couple thoughts as to how Tyr can make an exit from Faerun.