Does the punishment fit the crime? Sending someone home without pay for being 10 minutes late is excessive, unless the tardiness is a repeated violation and the employee has been told that repeated incidents could end in dismissal. Document all violations.
Did you discuss it first? Talk over the problem before you commit it to writing; it may help you draw a more accurate conclusion.
Are the facts clear? If everyone agrees on the events in question and you have proof that what happened violated company rules, your case is ironclad. But don't make a final decision to put it in writing until all ambiguities are gone.
Are you acting consistently? Review the discipline handed out over the past year. How does what happened then compare with the current situation?