@g0dgrave what manga were you reading? Tenma never has been a chad from page one till now.
And, even if we can point to his "philosophy" of all lives being equal as the root of his behavior, I think it's pretty evident that that principle is insufficient on its own. It is necessary to apply it wisely.
For instance, if all lives are equal, and he is taking an action (confessing) that puts his life at risk (likely death, but at least life imprisonment) in order to save another, he is trading 1:1. Lives being equal there is no argument based on that principle alone that justifies trading his life for another.
Furthermore, he should also be trying to factor in repercussions for his actions (obviously cannot be done perfectly, but still). If he confesses and Johan stays free, how many more people will suffer just so he saves Eva?