By considering only the last episodes on S5 there is only one plausible explanation for Red John knowing about Patick's "happy memory" and killing it.
First of all consider that Patrick said he never told anyone about that memory, so anyone that knew about it must have got from Patrick in some other way (written in a piece of papper on a notebook for an instance).

For all I know Kirkland is not likely to be Red John himself, also it would be too obvious.
But Bertram and Bob Kirkland on previous episodes have showed mutual respect and complicity. Therefore we can consider that all the information Kirkland got from breaking into Patrick's "office" is also available to Bertram.


One fact was still without explanation even with the end of the season: why did Patrick smile on episode 19 when he returned to his office and noticed that someone had broke in? Why wasn't he upset even for a second?

In my opinion Patrick planted the information on his room about the memory (to track the availability of his research to someone), so Kirkland got his men to run the search in Patricks room and passed on the information to Bertram, including a false (or not) one about a "happy memory". That explains the reason Patrick was not nervous when noticed someone had broke in and had access to all the sensitive information on his room. It was the plan from the start just like we see in several episodes.
The only two person that clearly had information on that room was Lisbon (when Jane let her in) and the men who stole information to give to Kirkland.
Red John can also be more than one person, still I believe Bertram is more likely.