Towards the end of episode 1, the detective from the local town gets mad at Holden because Holden tells him he doesn't know. After which Bill also gets mad at Holden for the same thing. However, as Holden's peer/mentor, Bill knows that they don't really know anything about the murder that was committed or why someone would do that. Why would Bill get mad at Holden for telling the truth?
Why would Bill get mad at Holden for telling the truth?
I don't think Bill had a problem with Holden telling the truth, per se, but instead, had a problem with how Holden behaved overall. The main issues Bill had with Holden [given the context of the OP] are:
#1) Holden didn't take heed of Bill's advice upon them first seeing the local officer in the diner, which was:
They always do this, just be a good listener.
#2) After being presented with the case information and Tench giving his opinion, Holden directly disagrees with the evaluation of Tench (who is much more experienced, and is also the superior).
#3) the truth didn't help the local officer, nor did it help the image of the FBI.
When first presented with the photos/information of the case, Bill talks as if he has a good understanding of what motivated the murder, with him quickly stating:
BILL TENCH: This falls clearly into the category of lust murder. By that I mean it's sexually motivated.
And, as he's saying this, Holden gives Bill a kind of look that can be interpreted as disagreement. When the local officer voices confusion about Bill's statement, Bill goes on to say:
BILL TENCH: It's more about sexual gratification through the annihilation of another.
From this, it's clear that Bill feels confident & knowledgeable about the circumstances surrounding this murder.
The situation becomes problematic when..
it's revealed that no trace evidence was left behind (suggesting premeditation), and when the role of the little boy was brought into question..
HOLDEN FORD: Why make the boy watch? .. Is this crime about the woman, or about the child?
At this point, Holden begins frustrating the local officer by taking a more "academic" line of inquiry by asking open-ended questions without providing any answers for them. At one point, the local officer thinks Holden is asking him specifically:
LOCAL OFFICER: You're asking me?
HOLDEN FORD: I'm just... posing questions.
LOCAL OFFICER: I get that... (leans back in chair and sighs)
Holden continues his "thinking out loud", but eventually, the local officer can't handle it anymore:
HOLDEN FORD: All I'm saying is that the broom might mean something...
LOCAL OFFICER: What?!
HOLDEN FORD: I don't know.
LOCAL OFFICER: Come on agent Ford, you've got fancy methods! What is the broomstick in the ass of a dirt poor single mom mean?
HOLDEN FORD: I don't know. I don't understand it. We can't help you with this.
At this point, Holden is convinced that the murder is not lust murder, but, by realizing the complexity of the emotions/motives behind the murder, knows that he can't help with the case. He then abruptly removes himself from the case, and leaves.
Once in the car,
the first words that were spoken, and spoken by Tench, were about Holden saying they're in the dark on this case (with Tench clearly thinking the opposite). Holden defends himself by saying:
HOLDEN FORD: It wasn't lust murder. It wasn't some random thrill-killer who was born bad, and it wasn't a panty thief who wanted to change things up.
As a rebuttal, Tench talks about how he took Holden under his wing, and how Holden is still inexperienced. After asking Holden about his personal life, Bill says:
BILL TENCH: So, next time you're along way from home, and you flip your shit, you find a payphone, and you tell it to your girlfriend. Okay?
This statement means that Holden shouldn't be so openly chatty about his thoughts, and that if he does want to be like that, don't do it to the people who are emotionally invested in a murder case.
So, again, Bill is upset with Holden because he's not responding to Bill's authority and seasoned recommendations, and because Holden is acting as if he's unaware of some of his actions/behaviors.
It's not that Holden was telling the truth per se ...it's the way he tells the truth.
After being belittled and embarrassed earlier in the police squad room the detective who was most dismissive actually reaches out to the FBI for help.
Bill has just told Holden that need needs to "know his audience" and make allies and friends with the various police forces they are trying to educate.
Holden initially make nice but then instead of sympathising more with the detective and explaining that their methods are at an early stage he just says "I don't know" and "We can't help you".
Bill tries to explain that they need more time but Holden cuts him off.
Yes, he tells the truth but they way he tells it means that he's not really listening to the advice he's getting from his mentor about making nice with others and by doing so drags down what they are trying to achieve.