use the weapons they have, fire them wherever they feel like it
Unlike Russia, Ukraine is not independent. That is, it is fully dependent on others.
So then it does not make sense for her to think in terms of "maybe better", or " what is morally justified". The only correct path for Ukraine, and this is evident even to a school child, is to continue conducting the war in such fashion so as not to lose the flow of aid completely.
I'm sure Zelensky and his team, are infinitely grateful that there exist such wonderful fellows such as yourself and Maf - willing to extend to them any credit of trust. Unfortunately, however, it is not with you that Zelensky has to consult (you would bless his every move), but instead with outfits like the US National Security Council. These places are filled with lawyers, that specialize in such silly things as Law of Armed Conflict. Then, they may also need to consult with people at the Department of State. The people at the Department of State, may just be pretty irritated that on top of covering for Netanyahu's genocide, they now need to cover for Zelensky's criminality as well.
So what do we make of the attack on Belgorod in view of the above? Several options are possible.
1) After being relatively passive in the aftermath of murdering Daria Dugina and sending a bunch of Russian neo-nazis into Belgorod under the cover of "Russian Liberation Forces" - Zelensky is back at it. This could mean he has despaired of receiving meaningful tranches of Western aid in the future, and will now give into the animal instincts of war.
2) Zelensky is being a politician. Russia's daily strikes in Ukraine point out his impotency in protecting his own population. On top of problems with his Chief of the General Staff, and the population at large, Zelensky's popularity is plummeting. Not just internally, but abroad as well. Instead of tiptoeing around his sweat stained sweater, the formerly meek Western politicians are beginning to find a voice again. What better, and cheaper method then - for Zelensky to quickly reinforce his support at home, and also show the West that there is no alternative to Zelensky when dealing with Ukraine?
At least, this may be Zelensky's calculus. It is in any case mistaken, as many of the policy decisions of this former comedian have been.