It means that result of what happened is that accord cannot take effect unless it is greatly changed. "Effectively" means that, while Trump did not actually cancel the TPP accord, that is the effect that his withdrawal of the United States from the accord will have. "Killing the accord" means that the metaphorical "life" of the accord has been ended. "In its current form" means the accord as it is now - it might be possible to bring it back if significant changes are made, but without those changes, it is dead.