Παλιόπαιδο θα γίνω

rotten child

for friends and you, till now

However, note that it the word "καλύτερο" is "lead by" : "το" ... therefore it would literally read "THE BETTER" ... subsequently, > "the best" ... you see ?!

Τώρα φεύγεις και είναι δύσκολη η ώρα

ένα αντίο μου πετάς και το κλειδί

"you throw a farewell and the key at me"

(=) rotten child I will become

(= I will break and will drink)

As, you probably know, Greek way of having "fun and dance" ("κέφι" , "γλέντι" ... probably you saw a Greek "oldy", " a black&white movie" from the 50's, 60's ) , would be to "break dishes" while "dancing" ... !!! ... lol ... an expression of "dance setting you free", "not caring about the trivialities of life", "allowing the soul to express freely, without any anxieties" ... so, it is partly a "heavy attitude" , but mainly a "not a care in the world" attitude ... and that is where the expression "τα σπάω", comes from ...

I tell you once and for all, (so you) to know it

a rotten child I will become, I am tired of giving

(=) by me if you come again, you will suffer

Your heart is empty (... ?!?)

(=) your life in the lie, you will squander (waste)

and I am sorry (sad) if later (on) you will regret (it)

you will not know, what it means to love