Should he pay back an interest-based loan?
Praise be to Allaah Who has guided you to the truth and has made you become religiously-committed. This is one of the greatest blessings that Allaah can bestow upon a Muslim.
If the loan involves paying interest when paying it back, then this loan is haraam and it is not permissible to continue with it. Allaah says (interpretation of the meaning):
“O you who believe! Fear Allaah and give up what remains (due to you) from Ribaa (from now onward) if you are (really) believers”
If it is possible to cancel this loan, then you must do so.
You have to repent and seek forgiveness, and feel regret for what you did, and resolve not to do it again. And be of good cheer, for Allaah accepts the repentance of His slave when he repents. The Prophet (peace and blessings of Allaah be upon him) said: “The one who repents of sin is like one who did not sin at all.” (Narrated by Ibn Maajah, 4250; classed as hasan by al-Albaani in Saheeh Ibn Maajah, 3427).
If you cannot get rid of this loan, then you have to pay it off. If you can get out of paying the interest without any negative consequences, then you must do that, because it is not permissible for you to pay interest (riba), for the Prophet (peace and blessings of Allaah be upon him) cursed the one who consumes riba, the one who pays it, the one who records it and the two who witness it. He said they are all the same. (Muslim, 1598).
If you cannot avoid paying this interest, then pay it because you are forced to do so, whilst hating it in your heart. Allaah does not burden anyone beyond what he is able to bear. It is sufficient for you to repent and regret what you did, in sha Allah.
The idea that these governments are financing the attacks against Chechnya is something which has to be proven. Even if it is proven, it does not mean that every dealing with them becomes haraam. Rather with regard to what involves helping in falsehood or in waging war against Muslims –as the questioner mentioned – or in things that are forbidden in sharee’ah, it is not permissible to help in that or to get involved in it.
And Allaah knows best.