Are you really that lost on the plot?
Lor'themar values the horde and it's survival matters to him. He clearly she's the obvious fact that sylvanas is an internal threat, if she came for Baine, how long before she came for him and his people?
And despite the obvious threat of war from the alliance, he unlike sylvanas understands diplomacy, if he can perform an action that creates a less hostile interaction with the enemy faction, it opens the door for diplomacy with them, and ending a war.
This is what a good leader does, if you can put aside your silly bias and think from a character perspective for a moment, you might understand why all these characters, saurfang, Baine, lor'themar, the others who are yet to side with them, are doing so.
This 'he's a traitor's boll*cks only comes from people to held on character fanaticism with the very obviously evil character here.