You raise a good point. I suppose for those who do not wish, for whatever reason, to introduce human reason into their faith (a near oxymoron to me but whatever), then it’s perfectly fine to treat all sin as the same. After all, that’s the “safe bet”, if all we have is the Bible as our guide.
Thankfully though, we Catholics aren’t so encumbered and I would submit, it’s a much better way to stay focused on what we are supposed to do, which is love and serve God. IOW, for me, if I had to live my life worried about every little sin as if it were equal to murder, I’d get very little done in the area of “loving and serving God”.
It must be difficult to live in such a way. Sadly, even some Saints in the Church have been saddled with such an unreasonable concern. It’s called “scrupulosity”. How can one live like that though? Only two ways I can see: 1. To believe that all one’s sins are forgiven already, even ones not committed yet or 2., to obsess about the smallest if sins, conflating even the smallest to the level of murder. Neither of which is truly a human way to live. At least IMO.
Thankfully we have more than Scripture to guide us in all aspects of the Faith. One is the reasonableness of our Faith. We are not only “allowed” to use the mind given to us but encouraged. Thus, such questions about which constitute serious sins and which do not are pretty much easy to answer. And in the few instances it’s not, we have the Tradition and /or the Magisterium upon which to rely.
For example, your question about “do not injure”, not being a specific command (in Scripture). This is true, so what to do with it?
First of all, it strikes me now that if one relies on Scripture alone to determine what is “sin” and what is not, I don’t think I’d want to run into such a “Christian”! After all, such a person wouldn’t ( or really couldn’t ) believe that physically injuring someone is a sin, for any reason, even if only in anger.
Secondly though, from reason it would stand that if “murdering” someone is a sin of grave matter, then beating someone would probably be one too! But even beyond that, in the Church, in many guides to the Sacrament of Confession, (called examinations of conscience), one question to ask oneself is, “Have you physically harmed another?”
If the answer is yes, then this is considered (at least possibly, depending on the severity of course), to be a violation of the 5th Commandment, “Thou shalt not kill”, and thus would be a sin as serious as taking a life.
Again, it all comes down to reason. If one isn’t willing to introduce reason into one’s faith, then that faith must become quite a burden (#2 above) or nothing of any real substance (#1 above). But to each his own I suppose.