切(き)るTo do something completely to the end, Utterly, Resolutely

Both 切る and おわる attach to verb stems to express finishing an action, but with different nuances. おわる simply marks completion, e.g. 使いおわった ('I finished using it.'). In contrast, 切る often implies using something up, e.g. バターを使い切った ('I used up all the butter.') or doing something to the very end with full effort, e.g. ゴールまで走り切った ('I ran all the way to the finish line.').

切る conveys the idea of doing something completely, which overlaps with one nuance of てしまう・ちゃう, but their usage differs. てしまう・ちゃう can imply that the action was unintentional or against one's will, which 切る cannot. In other words, 食べ切った simply means 'I ate all of it.', while 食べてしまった・食べちゃった also can have the meaning of mean 'I ended up eating it (even though I didn't intend to).', which 食べ切った cannot.