• In Vendetta, give away your position while waiting for General Amsel to show up.
  Fire your weapons while waiting for Amsel. If Reznov says above, you're good.
  
  
  
  
• In Their Land, Their Blood, do not kill any of the dying germans at the start of the level.
  Do not kill either of these germans. Ignore them and move along.
  
  
  
  
• In Eviction, mercy-kill the germans who are trying to surrender at the metro gate.
  Any attack besides throwing molotov at them will do. If Reznov says above, you're good.

If you follow these, Chernov will praise you as an indeed hero in his diary.

Any other actions you do in Soviet campaign, such as using molotov at random germans, killing any retreating germans, using flamethrower on the tank - don't matter. These three actions are the only factors that determines your action of mercy or brutality in Chernov's Diary.

I give my thanks to one of my Steam friends, Doktoberfest for solving this mystery. Credit goes to him.

If you have any questions, feel free to ask.