Why did G-d ask Avraham to sacrifice and slaughter his son?
Literally, he didn't ask him to slaughter him, just to bring him up on the alter as a sacrifice.
The whole thing was a test of faith whether he would listen to Gd with a good heart for such a painful test.
Here's a quote from the chovos halevavos shaar cheshbon hanefesh chapter 3 way#27
Know, my brother, that for the ten tests which G-d tested Avraham our forefather with, we would not be praising Avraham for standing up to these tests, if it were not the case that he received everything from his G-d willingly and with a good heart, as written: "And found his heart faithful before You" (Nechamia 9:8), and the generation who left Egypt did not become deserving of culpability and rebuke in the desert if not that they became angry and their hearts was not good with G-d and His prophet, ("even though they accepted everything that came to them and did not sin in speech, but their hearts were always grudging against G-d and they did not desire in His decrees but were like one who is forced to accept against his will" - TovLevanon comentary), as written: "But they beguiled Him with their mouth, and lied unto Him with their tongue" (Tehilim 78:36), and many times they would show their grudging and rebelliousness against G-d and the breaching of His covenant, as we find them constantly desiring to return to Egypt, or the like. Good bearing is a good character trait but one who bears (begrudgingly) out of force does not receive any reward for it, and he does not attain forgiveness from it ("G-d does not forgive his sins due to the suffering that came to him since he does not accept them contently but only out of being forced" - T.L.).