Uh … someone who says they didn’t murder is lying. Someone who keeps silent is not lying. Even the Catechism distinguishes between telling an untruth and keeping silence. This can be seen by its statements in terms of untruths never being moral but silence sometimes being not only permissible but the right and charitable thing to do. The Catechism even outright states something to the effect that some persons have NO RIGHT to the truth in some situations!

Wow. Your moral compass is in question if you feel that you can commit crimes and as long as you say nothing, it isn’t lying and you are free and clear.
ly⋅ing1 /ˈlaɪɪŋ/ Show Spelled Pronunciation [lahy-ing] Show IPA
Use lying in a Sentence
–noun 1. the telling of lies; untruthfulness.
–adjective 2. telling or containing lies; deliberately untruthful; mendacious; false: a lying report.
Origin:
1175–1225; ME; see lie 1 , -ing 1 , -ing 2
Related forms:
ly⋅ing⋅ly, adverb
Synonyms:
- falsehood, falsity, mendacity, prevarication. 2. deceptive, misleading, fallacious; sham, counterfeit.
I cannot find how keeping a crime a secret is considered truth. But the person is living a lie, being phony and being false and thus LYING by keeping it a secret.
That’s why calumny and detraction, gossip and slander are sins, even if the things you say are true.
Dear friend–slander is lying–spreading falsehoods. By definition it can’t be a true statement—ever. That would be an oxymoron.
slan⋅der /ˈslændər/ Show Spelled Pronunciation [slan-der] Show IPA
Use slander in a Sentence
–noun 1. defamation; calumny: rumors full of slander.
2. a malicious, false, and defamatory statement or report: a slander against his good name.
3. Law. defamation by oral utterance rather than by writing, pictures, etc.
–verb (used with object) 4. to utter slander against; defame.
–verb (used without object) 5. to utter or circulate slander
Gossip about things that are not true–is lying.
Gossip about things that are true–is just plain deceitful and disrespectful of whom you are talking about which is why those are sins.
As I said, it would be relevant if the murderer were claiming not to be a murderer, which they aren’t.
No–they are lying to the public. They are lying to authorities. Someone, somewhere–is looking for a suspect and this person is staying mute. They know who did it–they did. To themselves they admit they are a murderer. Their now-silent victim may or may not know who murdered them. But on a daily basis they live a lie for they have committed a crime and are selfishly keeping it to themselves.
The constitution may permit this–but morality does not and how can they be right with God without making right with the victim?
How one can be so selfish while being “really really really sorry” sounds like my kids when they don’t want consequences for hitting their sibling.