ORESTES: I’ll go. I’ll start to do this dreadful thing, this horror. Yes, I will. If it’s the gods’ will, I’ll do it. But I take no joy in it.

[ὈΡΈΣΤΗΣ: ἔσειμι: δεινοῦ δ᾽ ἄρχομαι προβλήματος
καὶ δεινὰ δράσω γε — εἰ θεοῖς δοκεῖ τάδε,
ἔστω: πικρὸν δὲ χἡδὺ τἀγώνισμά μοι.]

Euripides (485?-406? BC) Greek tragic dramatist
Electra [Ἠλέκτρα], l. 985ff (c. 420 BC) [tr. Wilson (2016)]
    (Source)

Orestes going to kill his mother, Clytemnestra, who was, along with the already-killed Aegisthus, the murderer of his father, Agamemnon.

Interestingly, earlier translations have him characterize the task as both bitter and sweet; later ones only speak of its bitterness.

(Source (Greek)). Alternate translations:

I go in.
Tho' I am entering on a deed that's fraught
With horror, I will execute the deed;
Thus let it be, if thus the righteous Gods
Ordain: altho' this conflict to my soul
At the same time be bitter, and yet sweet.
[tr. Wodhull (1809)]

I will go in; it is a dreadful task I am beginning and I will do dreadful things. If the gods approve, let it be; to me the contest is bitter and also sweet.
[tr. Coleridge (1891)]

I will enter in; but I am beginning a dreadful attempt. Ay, and I shall do dreadful things; but if this seems fit to the Gods, let it be; but the contest is for me [at once] bitter and sweet.
[tr. Buckley (1892)]

I will go in. A horror I essay!
Yea, horrors will achieve! If this please Heaven,
So be it. Bitter strife, yet sweet, for me.
[tr. Way (1896)]

Aye. So be it. -- I have ta'en
A path of many terrors: and shall do
Deeds horrible. 'Tis God will have it so. ...
Is this the joy of battle, or wild woe?
[tr. Murray (1905)]

I will go in; 'tis an awful task I undertake; an awful deed I have to do; still if it is Heaven's will, be it so; I loathe and yet I love the enterprise.
[tr. Coleridge (1938 ed.)]

Fine. I am going inside. Terrible the deed I shall begin and frightening the deeds I shall accomplish. If this is liked by the gods then so be it. My battle is bitter, not sweet.
[tr. Theodoridis (2006)]

I’ll go in.
I’m on the verge of a horrendous act,
something truly dreadful. Well, so be it,
if gods approve of this. And yet, for me
the contest is not sweet at all, but bitter.
[tr. Johnston (2009)]