Oh, it is true. Sometimes not doing X is not just unjust, but profoundly so. If it is (as Jesus made clear) morally correct to love one's enemies, then it's converse (not loving one's enemies) is clearly immoral.

If it is moral to feed and clothe the poor, then it's converse (not feeding...) is clearly immoral, too.

There is some conduct which has no morally neutral ground: one MUST love one's enemies; one MUST feed and clothe the poor. There is no moral defence for omission here: for neutrality, as it were.

I didn't mention the Fifth Commandment;  I mentioned the Gospel, in allusion to Jesus' teaching by word and example. If Jesus is the only supreme and perfect revelation of the nature of God (and I believe he is), then his example throws light on that commandment: intentional and deliberate killing of another, against his will, is wrong.