There are many reasons, besides general concern about conspiracy theories, that a Republican member of Congress would want to speak out on behalf of a former representative being unfairly attacked.
As members of a coequal branch of government charged with checking and balancing the executive, legislators might also feel duty-bound to rebuke flagrant abuses of the presidential pulpit, if only to deter future executive misbehavior. As public figures, they’d presumably all hope for defenders if the president were spreading thinly veiled murder accusations about them.
And even if they’re not concerned for Scarborough as a victim, they might have noticed that Trump’s attack on a political adversary did collateral harm to the still-grieving widower of the woman who died. “The President of the United States has taken something that does not belong to him—the memory of my dead wife—and perverted it for perceived political gain,” that widower wrote in a letter to Twitter asking that Trump’s tweet be removed. “I would also ask that you consider Lori’s niece and two nephews who will eventually come across this filth in the future.”