1.
to finish doing; to get done; to go ahead and do
やる plus the ～てしまう aspect marker. The neutral reading is completion: 'get the thing done', often with a sense of resolving to do it now rather than later.
2.
to mess up; to blow it; to do something one regrets
The same form takes on a negative emotional reading depending on context, especially in the past tense (やってしまった): 'I did it (and now I regret it)'. Contracted to やっちゃう / やっちゃった in casual speech.
やる + the aspect marker ～てしまう. The two senses share one form but are usually clear from context — present/volitional usually means completion, past usually means regret.