If a murderer using a knife as a weapon, how do they clear the knife from the victim without leaving marks?

A2A
Unless the killer was particularly “wedded” to retaining the knife (or they are simply very stupid) they dispose it as soon as possible.
Why?
1. Knives are rarely manufactured to be easily cleaned – Assuming that the killer knows what DNA is, then they would have to realize that it would be nearly impossible to remove it from a knife’s blade, hilt or haft. Keeping a knife with the victim’s DNA on it is essentially telling the authorities that you committed the crime.
2. Knives are cheap and plentiful – Unless you are a serial offender who loves $1,000 USD Randall knives or exotic custom blades, then purchasing knives which could be used effectively is as inexpensive as $7 USD at a truck stop or as much as $75 USD from an online web site. A knife simply is too easy to replace as opposed to a firearm to make retaining it worthwhile.
3. Many would simply wipe the blade off on the victim’s clothing – If we assume a fixed blade and not a folding knife or a switchblade, then we also have to assume a sheath or scabbard for the knife. If the blade is put away “wet”, then when it dries, it will “stick” to the inside of the sheath or scabbard and make it difficult, if not impossible, to draw again. Wiping the blade on the victim’s clothing prevents the knife from become stuck and also leave most of the victim’s blood and DNA at the crime scene.
Frankly, unless the knife is found intact and in the possession of the killer, and the victim is found before significant decomposition sets in, the question is irrelevant. If the killer uses a knife and quickly discards it, the chances of it being tied to him/her drops exponentially. If the knife was of cheap manufacture, and it rusts significantly before being discovered, it may not be identified as being the murder weapon.

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