A couple of points.
Gas Chambers were not made illegal, they simply stopped using them in favor of lethal injection, just like they stopped using Old Sparky and Firing Squads.
Now, lethal injection is a problem because no pharmaceutical company will sell them the drugs, and no doctor will stick the needle in. Something about that Hippocratic Oath.
This method is untested, which should make it suspect, and the man they are trying to kill with it already survived a half-ass attempt to give him the needle.
Now, ALL of that said, the problem with Capital Punishment overall is that it shouldn't be used because governments make mistakes. Cops lie about evidence because they are "sure" the perp is guilty, Prosecutors go with dubious cases because they need to keep a conviction rate up, and public defenders are often overworked and incompetent.
We shouldn't have a punishment that can't be revoked if we don't have a perfect system.