Right. But from very early days Christianity did exactly that. It has been what I would consider legalistic from the 2nd Century. As far as I can see Jesus specifically rejected that approach.
You can see in Paul why that might have happened. Roman culture was really a moral disaster. He (and many others in the early church) had to mentor converts. They did this by teaching contemporary Jewish standards. While Paul understood in principle the problems with Law, when put in the position of dealing with pagan converts, he took an approach that tends to look legalistic. He tried to mitigate that by saying that we really weren't bound by law, it's just that not everything is helpful. But when it came down to cases, he used the same "two ways" approach and sin lists as everyone else in the 1st Cent (not just Jews but the better pagan moralists).
That approach became even more extreme in the early church, with many Christians claiming that once you were baptized no forgiveness was possible at all. (At least in the church. Possibly God would ultimately forgive.) At this point Paul's practical compromise had been turned into a complete reversal of Jesus. Unfortunately this has continued. Rather than Jesus' emphasis on intent and what helps people, the Bible has been turned into a quote mine for rules. And now that's what 90% of the world, inside and outside the Church, think Jesus wanted.
