We live in a time in America where people scoff at the need for accuracy in matters of religion. Every road leads to God, we say (nobody knows why they say that, they just do. It is The Thing To Say). But the apostle knew that if you change the Gospel, you make it impossible to be saved from condemnation. It's like an old poem I once heard:
Johnny was a good boy
But Johnny is no more
'Cause what he thought was H2O
Hydrochloric acid, in other words. Just change a few letters, so to speak, and water becomes a deadly acid. In the same way, change the Gospel, and people die.
Paul says there is only one gospel ("which is not another", v. 7). He sees the gospel as such a life-or-death matter that he pronounces a curse down on anyone who preaches a false gospel. He pronounces a curse on the other apostles, on himself, even on angels, if any of them preach an inaccurate gospel.
These are not the words of a flatterer. They are not even the words of the modern church-growth guru. After all, "hot" is not cool, in modern marketing. The worst thing that can happen is that you be labelled a hater. A detached, knowing air is the hip way to be.
Which is stupid. If people are asleep and the house is burning down, they don't need you to be knowing, detached, and hip. They need you running up and down the halls, pounding on their bedroom doors screaming, "FIRE!" If you catch someone trying to hurt a child, a calm, detached response is wrong. You should get angry and hit the malefactor with a brick.
In the same way, Paul shows a healthy, hearty spirit of love when he wishes doom down on anyone (including himself) who preaches a false salvation message. This is love for others, not hatred of others. There is a good time to get angry. In the classic thriller The Third Man, the cynical black marketeer Harry Lime sells diluted anti-biotics to the hospitals . The sick children died in agony because they weren't receiving the real thing.
It was right to get angry at Harry Lime. It is right to get angry at people who change the gospel. It is right to guard the gospel, so that a world of sin-sick people can hear the true message of redemption.