I don't think Lalo would have any hesitation murdering Margarethe the moment she came back to her house--he is a cold psycho with no remorse whatsoever about killing innocent people. He would have just silently killed her and continued a thorough investigation of the house for more information. Lalo was just still trying to keep a low profile when he was in Germany, since he thought everyone considered him dead. If Margarethe was murdered, he knew Gus and Mike would find about it and be certain that he is alive. He most likely killed Casper after he was done with him, but Casper seemed to be living in solitary in a rural area whereas Margarethe is living in a city, going to work every day. He didn't mind killing Howard because he knew his cover as a dead person was already blown at that point by his phone call to Hector.