I would suggest suo periculo egit/fecit (i.e., He did at his own risk).
the phrase suo periculo was used by several classical Latin authors. It is also very obiquitus in Digesta Iustiniani by Iustinianus, from there it evolved in our day legal jargon.
I found this phrase in Sebastian Castellio translation Jos. 2:19.
Quisquis autem extra domus tuae fores exierit, suo periculo fecerit, et nos aberimus a culpa. Quidquid domi apud te erit, id si quis attigerit, nos luemus.
It must be noted that Sebastian Castellio's is usually not literal in his translation and he actively seeks classical equivalents. This verse is a good example of this as he translated the literal "his blood shall be on his own head" (i.e., he is to be responsible for this result) for suo periculo fecerit.
This phrase is more appropriate for cases of negative consequences. It is a handy way of saying "you are to be blamed" which is basically a short way of saying what we read in the title: "you should take responsibility for your actions, and accept the consequences if you don't?".
We can turn this into a more general motto, which is unfortunately not attested:
** Quidquid (tu) facis, tuo periculo facis.** (Whatever you do, you do at your own risk).
Other than that, my hunch is a nice motto expression can be found in Publilius Syrus and the distichs of Cato (which is later 4th century). Here example from the later 2.21:
- Quae potu peccas, ignoscere tu tibi noli;
Nam crimen nullum vini est, sed culpa bibentis.
Whatever error you do while drinking, don't pardon yourself,
For there is no fault in the wine, but in the drinker.
Doing some more search with "quidquid facis/agis" I found a nice proverb of unknown origin that encapsulate this general idea of responsibility.
Quidquid agis, prudenter agas et respice finem. (whatever you do, do it wisely and consider the outcome)