From SN 42.3:
When a warrior strives and struggles in battle, their mind is already
low, degraded, and misdirected as they think:
Yo so, gāmaṇi,
yodhājīvo saṅgāme ussahati vāyamati, tassa taṁ cittaṁ pubbe gahitaṁ
‘May these sentient beings be killed, slaughtered, slain, destroyed,
His foes kill him and finish him off, and when his
body breaks up, after death, he’s reborn in the hell called ‘The
From the sutta quote above, we see that the warrior who strives (ussahati) and struggles or exerts himself (vāyamati) in battle, is already deep in the low, degraded and misdirected mindset of killing.
Similarly, just having the fleeting thought of hurting the Buddha, killing an arahant, killing one's mother, killing one's father or causing a schism in the Buddha's sangha, is not sufficient to be considered an incurable action.
It's an incurable action only when one strives (ussahati) and struggles or exerts himself (vāyamati) to commit those heinous acts, regardless of whether he has actually been successful or not, just as in the case of the warrior in battle.
So, yes, even an unsuccessful intentional and deliberate attempt to perform these wicked acts would be incurable.
The OP asked in the comments to the effect of: What about Angulimala who killed many humans and attempted to kill the Buddha, but eventually became an arahant?
When Angulimala attempted to kill the Buddha, he did not know that it was the Buddha or an arahant. He only saw a monk walking alone along the road.
So he did not have the intention to kill the Buddha. He only had the intention to kill a monk.
Please read the story of Chakkhupala in the commentary of Dhammapada 1 and also the quote:
"Intention, I tell you, is kamma. Intending, one does kamma by way of
body, speech, & intellect." - AN 6.63
Karma is based on intention in Buddhism. Karma is not a universal system of justice.