In the poem, "My Last Duchess," Browning creates a narrator who conveys the distinct displeasure he feels at the perceived infidelities of his wife. There is some question as to the sanity of the narrator as he can calmly talk about the beautiful picture of his wife, then transition into tales of her betrayal, and then calmly explain that he was not willing to compromise so he gave the words to have her killed, all the while keeping the next duchess waiting downstairs.
With the poem, "The Laboratory," there is little doubt that the narrator is crazy. We, as readers, can almost see his hunched shoulders and intense concentration as he ponders what poison will be best to use when killing his wife. As readers, we still don't know if the indiscretions were real, but we have absolutely no doubt how the wife was killed--by his own hand.
Both husbands are angry and bitter, but we get the feeling that the husband in "The Laboratory" is hurt more by the emotional betrayal. While the husband in "Last Duchess" is more angry at the insult of the betrayal.
I just wanted to comment on the previous answer..
They seem to think 'The Laboratory' is about a man killing his wife, but I think it is clearly about a wife who wishes to poison her husband's lover in order to punish her husband for committing adultery. Phrases such as "Not that I bid you spare her the pain; Let her death be felt and the proof remain", "He is sure to remember her dying face", and "You may kiss me, old man, on my mouth if you will!" do not make sense otherwise.