I read the whole story about the death of Tai and Maria.
Tai was torchered by Locust soldiers as they knifed him on his back making a Locust symbol on his back. Locust had also punched him, kicked him, and starved him. As he had slept over time in the prison chamber, Locust specials performed tests on him, causing him to lose his memory and make him kill himself if he held a weapon. Tai finally gets saved by Marcus and his squad, Marcus hands him a shotgun, the memory of killing himself kicks in. Tai loads the gun, and shoots himself in the head.
Same story with Maria. She gets captured during Emergence day causing her to be held in Locust prison during the war. In the second Gears of War, Dom goes out of his way looking for Maria as he fights through enemy lines. Later in the game, Dom and Marcus find the prison pod Maria is located in, after fighting off Locust Guards. Dom halucinates seeing Maria as if normal, but in reality, she looks like she has been tortured, tested on, and starved. Dom says to his clueless wife that he loves her, and will never forget her. As he pulls out his revolver, he says his final words to Maria, then camera goes to Marcus walking away from the scene, hearing the gun shot, stopping, then walking on.
There you go if you wanted the whole story on how they were tortured and stuff.