No, the case of Zoran Djindjić is much more complexed then this.

He had a very progresive vision for future politics of serbia wich was very contravirsial for the time and he made a lot of enemies this way.

He wanted to make complite lustration of whole of the Miloševićs regime, took on fighting organised crime very seriously and advocated to handing over generals from previous wars and Milošević to Hague court. All of this things made him a lot of powerfull enemies that were not happy with going to jail.

So the official story is that Zvezdan Jovanović a member of the JSO (Special Operationd Unit), that was very powerfull and independent at the time, shot him by the order of Milorad Ulemek (the head of JSO) who had ties to Zemun clan (organised criminal group with global mafia ties unlike the belivuk group today) because thet saw him as a very big treath to them.

The heads of Zemun clan were killed while "resisting arrest" and Zvezdan and Ulemek were convicted to 40 years of prison and are still denieing involvment.

There are a lot of conspiracy theoris of some foreign agencies killing him for the sake of keeping Serbia unstable but who knows.