It’s not really about ‘better’ or ‘worse’, rather the most logical solution to deal with a series of murders that can only finger the blame at the wrong people. Face it: it would have been Eva’s and Rudolf’s word against each other, the latter of whom had mountains of enemies who would love to see them rot in prison, versus just making all the nightmares of the night going away by blowing up the island. Kyrie made the cold logical call of removing all possible incriminating witnesses and deliberately made Rudolf also partake in at least one murder so he would be equally complicit and would have to shoulder the burden of killing all the rest of the members of his family.
It’s evident that in part the desperation for a quick solution to their problems, and their greed for the billion yen played a huge part in that thought process as well. Furthermore, if Rudolf and Kyrie hadn’t made the call they had, not only would suspicion fall on the most suspicious of the lot, but so would the ensuing imprisonment that would no doubt follow. Also, if they didn’t make the call they did then, it would only serve to stab them in the back, since the possibility also existed that other ‘survivors’ may also come to the same conclusion as them. Everyone is evidenced to have such a thinking capacity as evidenced from the preceding arcs.
Basically, Kyrie and Rudolf were protecting themselves. They were surrounded by potential killers and all of whom would gladly take all of the billion yen over the lives of people they weren’t even close to in a heartbeat. Being outsiders, we judge them as immoral since we can’t imagine doing the same, since we don’t put ourselves into their shoes generally.
It’s also naturally naïve, no offence here, to believe that if Kyrie and Rudolf hadn’t carried out the massacre, the tragedy wouldn’t have occurred in the first place. Make no mistake there, the prior arcs have already demonstrated the capacity of all of the living siblings in the VIP room, barring Rudolf, to be capable of doing the same. Also, Episode 3 demonstrated Eva’s capability of being the mastermind behind the entire incident.
Everyone in the Ushiromiya family has some amount of blood on their hands, directly or indirectly, in all of the arcs. It’s just because we’d rather Beatrice be the perpetrator of the crime, a belief seemingly supported well by the question arcs up until this point. Because this was the actual events that took place, it’s only natural that we wouldn’t want our beloved characters to be actually responsible for the entire series of murders.
Of course, even Kyrie and Rudolf meet their just ends at the hands of Eva, who manages to slip away (and Battler, who was miraculously unaware of all of these events). Not to mention Eva being herself a murderer having to face countless accusations of being the perpetrator behind the entire Rokkenjima murder. I could be wrong here, but it’s entirely possible all of the murderers face this stigma in the VN as a means of telling the readers that something that can be called a justifiable murder doesn’t exist. Of course, with regard to Eva, some may argue that she was unable to voice the truth because of the hurt it would cause Ange, however, one may also view that as the burden of a lie on the murderer, who must profess his innocence by implicating another, or in this case, implicating several parties due to a lack of inculpatory evidence against said murderer.
It’s a long answer, but I hope this clears people’s doubts about the logic behind Kyrie’s actions. There’s no Golden Land at the end of Episode 7. But to quell the sadness of the readers towards such a dark ending, I’d imagine Ryuukishi07 wrote Episode 8, to end the series on a semi-positive note.