Here's a walkthrough if you're having some trouble:

Normal ending is when you guess who's the killer but you don't have enough points for happy ending. It's the same for every character.

HAPPY ENDINGS (you need to collect 4 points to get those endings):

When you'll get the choice to knock to the front door or not, you will meet three of incoming characters which are:

Szczur - pick "Funny"

Lulek - pick "Temptation"

Lala - pick "Reassuring"

Another 2 points you'll get by choosing the same person for your first interrogation (it doesn't matter if you go through any of given questions there at all, just pick 'em).

In case of Metka:

+2 points for choosing her in your first interrogation,

+2 points when you ask about "Personal info", go through the dialogue and choose the answer "Yes".

BAD ENDINGS:

Arrested:

When you'll get a choice about the letter, choose "Show it to the policeman". That's all.

Cookied:

Only when LALA is the killer, can you obtain this ending. In both FRIST and SECOND interrogation she'll ask you if you want to taste her homebaked cookies, simply agree (it's on the question about the PLACE).

When you'll make a mistake in voting and won't choose the real killer, you'll unlock bad endings for each of the four characters (Lulek, Lala, Szczur, Metka).

HINTS IN THE DIALOGUE (meaning when the code changes if the character is the killer):

Interrogation 1:

Lala:

• "Place" - eating poisoned cookies,
• "Alibi" - her mother will say she wasn't home,

Metka:

• "Alibi" - she'll say she doesn't want to help you,
• "Relationships" - when askin about Lulek, she'll get mad and tell you all about how she hates him.

Szczur and Lulek won't give you any hints here.

Interrogation 2:

Lulek:

• "Alibi" - he can't prove his innocance

Lala:

• before questions - she's smile at you, otherwise she'd look at someone else,
• "Place" - cookies, again,
• "Alibi" - she won't show you the date

Szczur:

• "Alibi" - he states that Metka can give him alibi,
• "Secret" - he confesses about being the killer

Metka:

• (even if you didn't pick her, Lulek can tell you in "Relationships" that he still loves her, otherwise he'd say deny it)

Interrogation 3:

• Lulek - he'll ask if you suspect him, otherwise he'll be talking about the past
• Lala - she'll ask if you suspect her, otherwise she'll say she isn't
• Szczur - he'll admit it, otherwise he'll ask you for forgiveness
• Metka - she'll ask you to stop joking, otherwise she'll smile

Important note: Lulek lies for fun so even if someone else claims he lied, he more probably did it for no reason.

Hope it helped!