Are loving-kindness and compassion (metta and karuna) special kind of attachments? To alleviate someone from suffering, Buddhism teaches one to practice mettā
karuṇā. But it seems to me that to thinking good about someone and wish them the best, we need to have a better version of the suffered person, so that we can disagree with their current version. If we don't have that image, then we wouldn't say the suffered person is suffered at the beginning. Thus, it seems correct to say that in order to practice metta and karuna, we need to have an attachment?

Since this attachment is necessary, then I think it's fine to have? For example I ask this question, thus I have some attachment to it, and that's fine. I think this is related to the conventional truth and absolute truth. 

So is it correct the in order to practice metta and karuna, we need to have an attachment to begin with? 

Edit: One can simply say that the better version of that person is just a ideal fabrication of them (because they are not actually like that), or just the good side of them that they always have but not shown yet. In either case, I think it's necessary to assume good faith. Would that assumption be an attachment?



Related:
• Is radiating loving kindness increase attachment?
• How to view people with metta and karuna?
• Is there any source saying that Buddhists can temporarily form relationship to help people?
So is it correct the in order to practice metta and karuna, we need to have an attachment to begin with?


Well, attachment is a default software built into the core kernel of every human's "operating system", so whether we need to have it or not is kinda irrelevant. And no, there's no need to come up with some fabrication or some altered reality about the other person's personality in order to cultivate metta/karuna. It's a truth  that both evil and goodness co-exist within every single human being, it's just that the relative ratio is different from person to person. So it's not a fabrication to focus on a person's 'good side', because s/he does have a good side! So the beauty of metta/karuna cultivation is that one can start training right away, even with their existing attachment at the beginning stage. But as one proceeds to higher, deeper, more subtle levels of the Path, the attachment will naturally subside and eventually goes away completely. Once it's become second nature, Metta/karuna at that stage will no longer involves any kind of attachment at all.