People can have whatever opinion on whether the shooter was right or not. But the person who modified a gun, shot his target thrice in the middle of a city, left a James Bond style message on the bullets, and rode off into the sunset without leaving a single clear photo on any nearby camera is obviously not the man who got arrested eating a meal in broad daylight with a gun and a written confession conveniently packed in his bag.
When modern-day Ukraine was formed in 1990, it was majority Ukrainian, but with a sizable Russian (and smaller Romanian and Polish) minority. Over the next twenty or so years, this minority voted for parties and politicians that favoured stronger ties with Russia. In contrast, ethnic Ukrainians supported joining (or at least aligning with) the EU. This conflict came to a head in 2014, when the pro-Russian government was overthrown by pro-EU protestors. Relations between the two groups have worsened since then, leading to pro-Russian militants seizing power in the (Russian majority) Donbass and Crimea, and joining Russia.