ScriptSig field during P2PKH transaction signature
According to the accepted answer from How to redeem a basic Tx?, the field is temporarily set with the ScriptPubKey field from the UTXO we want to redeem (in this case a P2PKH ScriptPubKey with the RIPEMD160 of the previous address). However, according to VButerin's pybitcointools, the field is set with a P2PKH ScriptPubKey with the RIPEMD160 of your own address (the one you are using as a source, and which private key will be used to sign the transaction).


You're saying the same thing twice here.

A transaction T1 existed with an output T1:0 which has a scriptPubKey P2PKH(K1). This is the transaction that credited you. It resulted in the creation of a UTXO with script P2PKH(K1), and your own key is K1.

You're trying to create a transaction T2 with as input T1:0 and an output T2:0 with scriptPubKey P2PKH(K2). This is the spending transaction, which you're signing for with K1. It sends money to K2, but consumes the UTXO T1:0 which has script P2PKH(K1).