Kalki is the Avatar of Kaliyuga. Though Lord Venkateswara is born to fulfill some of the promises he made in previous incarnations but he didnt kill any evil.
Logically Kalki will fight with Kali who is the head of Lust, gold etc..
currently kali is not visible to the physical world. so Kalki will be too.
Kalki will fight with Kali in a different dimension ...
Who said Kali Yug is bad ?
Ved Vyasa himself said "Kali Sadhu:" - meaning Kali is great
Each Yug has its own methods of attaining Bhagavan. Kali is the easiest.
dhyayan krite yajan yajnais
tretayam dvapare arcayan
yad apnoti tad apnoti
kalau sankirtya kesavam
Whatever is achieved by dhyana in Satya-yuga,
by the performance of yagna in ...
There's for sure death and decay at the end of Kaliyuga- which is full of faults, passions, miseries and confusions. This Tamoguna, Rajas once vanished, Sattva gunas can attain peaks giving rise to beings bounded by righteousness- Satyuga begins with the end of the Kali age.
At pralayakaal, Rudra performs samhar tandava along with Mahakaali.