Christ's teaching here is preliminary to the Sermon that follows. In the coming Sermon we will read a better preserved version of the Sermon on the Mount from Jerusalem, called here the Sermon at Bountiful. But this explanation of doctrine is given by Christ first. The foundation of doctrine of the oneness of the Father, Son and Holy Ghost, the conferral of power to baptize, and manner of baptism come before the great Sermon. First we receive the instruction to avoid disputes. These disputes lead to contention that lays the foundation for anger between men. This doctrine is so foundational that Christ covers it before any other teaching. Therefore, you should realize its importance.
We will be captured by hell if we do not understand and follow these teachings. Though they are Christ's very first instructions, we almost never discuss them. You may want to re-read these verses again, and realize their fundamental importance.
Christ is saying it is "evil" to do more or less with His doctrine. It surely is, for ignoring, altering, omitting or enlarging leads to evil.