Firstly, we need to be a bit careful as the Milindapanha was recorded after the First Buddhist Council and is not considered by some to be part of the Tripitaka.
https://www.accesstoinsight.org/noncanon/index.html
The question posed by King Milinda is whether the Buddha burnt out all evil in himself when he became a Buddha and if he had indeed burnt out all evil how was it that he hurt himself after becoming a Buddha?
This is a bit of a confusing question. One does not experience pain because of evil per say but because of the result of such an evil (thought/deed). So even though the Buddha does not have any evil within himself he can experience results due to past evil he had done.
These 8 causes are mentioned in SN36.21 (Sivaka Sutta), but I could not find them in AN10.60 (Girimananda Sutta)
In SN36.21, the Buddha’s opinion on some claims made by Brahmana’s that “whatever a person experience’s, all that is caused by what was done in the past” is clarified. The Buddha refutes this claim that everything that happens, is caused by something done in the past and gives 8 causes, one being kamma-vipaka. It has to be understood here that “what has been done in the past” does not refer to kamma-vipaka, as the Buddha says that, it is wrong to think that everything happens due to what was done in the past because kamma-vipaka can also be a cause.
It seems like the answer to the question in SN36.21 has been given for the question asked by King Milinda, which is a bit mis-leading.
Though Ven. Nagasena says that the wounding of the Buddha’s foot is not because of bad kamma this is not what is said elsewhere. “Even to the Buddha himself it happened that, as a result of former deeds, Devadatta was able to cause him a slight injury.”
Also in this same light when Angulimala (an arahanth, who has burnt out all evil within himself) was hit by rocks by people while walking on the road, can this also be only due to external agency? The Buddha says here, “Bear it, brahmana, bear it, brahmana! You have experienced here and now the ripening of kamma whose ripening you might have experienced in hell over many a year, many a century, many a millennium.” From this it can be seen that both external agency as well as kamma-vipaka played a part in Angulimala getting hurt.
This also shows that one does not need to repay all the kamma to attain nibbana. If one has to repay all kamma to attain nibbana, Angulimala would not have been able to attain nibbana until he repaid all of it in hell. The level of vipaka he would have had to experience in hell cannot be even compared to getting hit by stones and rocks.
AN10.217 and AN10.218 does not say that all kamma needs to be repaid. In AN10.219 it is said “The noble disciple understands: ‘Whatever bad deed I did here in the past with this deed-born body is all to be experienced here. It will not follow along.’” This is consistent with what the Buddha told Angulimala.
However, I need to find a sutta reference to kamma being the root / primary cause. If anyone does come across something please let me know.