As reported, the attack, allegedly carried out by a member of a Ukrainian paramilitary group, happened at a protest against a constitutional amendment that would devolve more power to local governments. The idea isn't controversial, and it was a condition of the agreement that five political parties reached last year to form the current governing coalition. Minsk II requires Kyiv to legislate devolution before the end of the year.
The problem is that Ukraine's President Petro Poroshenko is under pressure from his Western allies to abide by Minsk even as Russia's President Vladimir Putin violates it. Mr. Poroshenko's critics object to amending the constitution during a war and say the amendment as written risks giving too much autonomy to Donetsk and Luhansk, the eastern regions occupied by Russian-sponsored rebels.
Mr. Poroshenko's coalition has started to fracture, with the smallest of the five parties in the government withdrawing Tuesday. The remaining parties still enjoy a majority, so a collapse isn't imminent. But the episode illustrates the extent to which Mr. Poroshenko is paying a mounting political price for being the only leader honoring his end of the bargain.
Meanwhile, Mr. Putin's cease-fire violations go unpunished, as the U.S. and Europe stall on providing Kyiv with lethal aid that would impose higher costs on rebel incursions. The European Union this week agreed to extend its sanctions on Russia for another six months only after hemming and hawing over the possibility of a shorter term. This cease-fire has a winner, and it's not what remains of a free and democratic Ukraine.