There's a difference between booing (which very much gets the point of Trump's disapproval across to the rest of the world as it should) and shouting "lock him up", which is an expression of mob mentality and which is, indeed, not really something you'd want any crowd (liberal or conservative) to be shouting.
And I say this as someone who's very much hoping that 1) Trump loses in 2020, 2) gets indicted for the various crimes his office is currently shielding him from, 3) gets found guilty in a trial and then 4) gets to spend some time lounging in a real (non-Trump owned) federal hotel. (especially since he's been openly rubbing in everyone's faces about how he can openly break the law without getting prosecuted for it) But in a society that values the law, in order for step 4 to occur, step 2 and 3 need to take place and they're not optional. I get that "Lock him/her up" is a catchier slogan to shout than "indict him/her" or "put him/her on trial", but it also carries the subtext that whoever shouts it doesn't care about those preceeding steps and just wants some quick wild west justice. Which makes it kind of an unsavory chant no matter if Hillary or Trump's the target.
If Trump loses, the next Democratic president might have to determine whether to proceed with the socially disruptive process of allowing the next Attorney General to bring criminal charges against his/her predecessor. Pelosi and Schiff are taking great pains to approach the impeachment inquiry with a narrative of "We're gonna dig up the facts, see where they take us and act accordingly and we're not gonna determine guilt beforehand" (despite the fact they both know Trump's guilty) rather than going with "We're gonna throw this mofo out" because they recognize the importance of due process. For the same reason, Trump's successor will have more room to pursue charges against Trump if the liberal base doesn't make a habit of shouting "lock him up" at every rally from now up to november 2020.
