In act 3, scene 1, what does Shylock mean when he says, "The villainy you teach me, I will execute— and it shall go hard but I will better the instruction" to conclude his speech?

Shylock makes the speech in Act three scene one of Merchant of Venice to defend his right to ask Antonio for a pound of his flesh. He claims, strongly, that Antonio has gone out of his way to ruin his livelihood and insult his people; and all because he is Jewish.

He hath disgraced me, andhindered me half a million; laughed at my losses,mocked at my gains, scorned my nation, thwarted mybargains, cooled my friends, heated mineenemies; and what's his reason? I am a Jew.

He rightly points out that in every way bar religion and some cultural differences, a Jew and Christian are the same. They eat, they drink, they sleep. "If you prick us do we not bleed? ... If you tickle us do we not laugh?" In this respect he says the Jews have the internal need to seek revenge for wrong doings as much as any Christian. So why shouldn't he ask for a pound of Antonio's flesh when Antonio has treated him with such disregard? Far from being being unchristian, it is what Christian society has taught him to do.
Therefore by saying "the villainy you teach me, I will execute-and it shall go hard but I will better the instruction," Shylock is saying that he is doing what a Christian would do to him in a similar situation. The only difference is that he intends to do it better.

At the end of his Act III, Scene 1 speech in which he argues a Jew feels things just as a Christian does, Shylock tells Salarino he will treat the Christians by the example set for him. In fact, Shylock says he may even outdo the "Christian example" he has been taught.
Shylock, in anticipation of the execution of his revenge against Antonio, hopes to "better the example" of what he calls Christian retaliation. For, earlier in his speech to Salarino, he has said sarcastically,

If a Jew offends a Christian, what’s the Christian’s kind and gentle reaction? Revenge.

Therefore, if Antonio's merchant ships are truly lost, Shylock fully intends to outdo the Christians' examples of revenge. He will demand the pound of flesh from Antonio, who has lost him many ducats in interest that Shylock could have acquired from loaning money if Antonio had not intervened repeatedly. Since Antonio lends money without charging interest as "a Christian courtesy," those in need approach him for loans instead of going to Shylock.