In 'murder will out', meaning 'murder will become public (knowledge)' will is, once again, a modal auxiliary. In modern English, it is very rare to use a modal + full verb combination without actually using the full verb in this way, but you will occasionally hear, "I must away".
A simple test is the third person -s ending. If it's there, it's a full verb, as in: He's a faith healer - he wills people well.
If there is no -s ending, it's a modal: Death will come when it will come.
This is not an infallible test if you are one of those who still use the present subjunctive, in which full verbs do not end in-s in the third person singular. Here is an example, using will=bequeath: I recommend that he will half of his estate to to his surviving sons, and half....
The full verb uses DO for questions and negatives, the modal does not:
Murder will not out.[STRIKE]Murder don't will out[/STRIKE]. - Modal
Death will not come. [STRIKE]Death don't will come[/STRIKE].- Modal
[STRIKE]He wills not people well[/STRIKE]. He doesn't will people well. - Full verb