The extra-judicial killing of a tyrant, for the crimes they have committed in their abuse of power, is the sort of moral question that moral philosophers tend to shy away from. There are, however, methods by which one can work through this sort of question.

First -- one must recognize that there are multitudes of moral stances that one may adopt, and the answer can vary significantly between them case-case. To do valid moral thinking, one must therefore think through amoral question from many of these stances, and should only adopt actions that are supported by all or most of the "best" of them.

Second -- there are worse and better moral stances. All of them are endorsed by advocates. But a reasonable standard for "worse" is peer opinion. The "worse" ones, are those which the non-adherents think they lead to particularly bad moral outcomes much of the time. Among these worser ones would be Legalism (in which the Tyrant would, as lawgiver, always be "in the right" whatever offenses he committed); Darwinian rationalizations for "alpha" behavior; and references to theological or cultural legalism. Under many of these worser stances, tyrranicide is always wrong. But as worser stances, their views are mostly irrelevant.

Third -- some moral stances are simply too diverse to be of much use. "Wisdom traditions" often encode good pragmatic wisdom, but they are very culturally specific, and some cultures will have useful guidance on a question like this, and many others will not. The historical Virtue Ethics have the same problem -- there are too many virtue sets that have been followed, and the answer depends on the specific set one is working from.

There are three primary stances that are widely seen as "better": Utilitarianism, Rights Ethic, and a pursuit of Truth and Love as overriding virtues. One can examine these three stances, and arrive at a pretty good understanding of how to behave in a particular instance. In addition to these three, there are two darwinian-inspired perspectives that are less objectionable to non-Darwinians, which can also be examined -- Eusocial ethics, and Deep Ecology.

Rights Ethic is simplest here, as outcomes and motives do not matter. Self defense, and defense of others from abuse, justifies Tells tyrranicide.
Rights Ethic also advocates for a We the People justification for governance, which Gessler was violating, so Gessler was also guilty of violating his social contract with the governed.

In Utilitarianism, it matters what happens NEXT. IF tyranny was just the nature of governance of the time, then there was not a crime or a problem on Gessler's part, just SOP bad governance. BUT -- if there was particular opportunity to institute much better governance, which apparently from history there WAS, then EVEN IF Gessler was not a particularly bad ruler, EVEN THEN, his murder and overthrow would be morally justified. utilitarianism therefore endorses his tyrranicide.

Truth is not a big driver of appropriate action in this case, but Love ethics has a lot to say. Killing someone is almost always wrong in Love ethics. The goal should always be to reach out and try to establish a caring connection, and try to convert Gessler to a better person through a transformation of the heart. Love Virtue ethics seem to work better if one accepts personal immortality, and the relative unimportance of exactly when one dies in this world. Under Love virtue, tyrannicide is wrong.

Under Eusocial Ethics -- it matters what happens to the stability and effectiveness of the society with or without the tyrranicide. The Swiss system became stable and highly productive, much more so than the feudal systems around them, hence eusocial ethics would endorse the tyrranicide.

Under Deep Ecology, the health of Gaia is the key question. As humans are the reasoning arm of Gaia, we are needed to protect Her from events She cannot predict or prevent without planning. The outcome of a governmental dispute in a tiny corner of Gaia, and the possible deaths of a few humans in the process, is of little import in Deep Ecology. BUT -- the principles of unchecked power with no moral or other consequences, which is exercised by Gessler, is anathema to Deep Ecology ethics. Overturning them, and the development of a Canton government with a concept of Responsibility to Constituents -- THIS matters a LOT to Deep Ecology. Deep Ecology would endorse the application of moral consequences to Gessler.

Four of these five better moral stances all support Tell's Tyrranicide.