Why did Hashem instruct Moshe already now to mention the killing of the firstborn to Pharaoh?
Rashi: To soften his stance, when he would hear him beginning to refuse. He warned him already at the outset 1 , what he could expect should he refuse, on account of the severity of the punishment.
Ramban: This was not a command; He was informing Moshe that there would come a stage where Pharaoh had refused so many times that he would tell him about the impending Makas Bechoros as a result of his refusal. 2
Seforno: In fact, Makas Bechoros was the only plague that would come as a punishment, Midah Keneged Midah 3 for refusing Hashem's firstborn to go and worship Him. The first nine plagues were to prompt Pharaoh to repent and to let Yisrael go and worship Him primarily based on the love of His goodness and in awe of His Supremacy, or at least like servants who are afraid of their master's ability to punish them. 4
Unlike a human-being, who, afraid that his enemy will stall his plans, does not reveal them in advance, Hakadosh-Baruch-Hu, who is All powerful, can reveal his plans, because nobody can prevent them from materializing, as the Pasuk states in Iyov (36:22 [Rashi]).
Ramban: Since we do not find that Moshe told Pharaoh about Makas Bechoros until then (See Bo, Shemos 11:5. The Ramban interprets "va'Tema'en Leshalcho" in the past tense (as a statement), as opposed to Rashi and the other commentaries, who interpret it (as a warning) in the future.