These talking heads are so ignorant sometimes -- they don't seem to understand that premeditation is something that the state has to prove beyond a reasonable doubt.
The circumstances that point to premeditation cannot just be left to chance - Juan would be negligent if he just assumed "well, the jury already knows she's lying, so I don't need to prove every single lie".
Beyond a reasonable doubt means you put in every single bit of evidence you have to make your point, and you never make an assumption that a jury will automatically disregard her lies and come to the right conclusion without explicit, detailed proof.